Changing the popup Calendar size in WPF DatePicker - wpf

Question

I've edited my DatePicker template (after right click -> edit copy) and successfully resized the icon and textblock, but can't find an option to change the popup calendar size without resizing the whole DatePicker.
I've seen other answers about using ViewBox around the <Calendar> tag to set the width of the calendar. However, in my template there is a <Popup> - if I use hardcoded width it doesn't show. If I encapsulate the <Popup> in a <Viewbox> it resizes the whole DatePicker (the textblock and button).
What I get right now:
Very small calendar
If I do <ViewBox Width="300"><Popup /></ViewBox>, the whole DatePicker row grows to 327 in width (and button doesn't work).
Setting hard width and height does cut off the calendar, but doesn't grow it (ofc).
The template tree (9th row from bottom):
<ControlTemplate x:Key="DatePickerNew" TargetType="{x:Type DatePicker}">
<Border BorderThickness="1" Background="{StaticResource LightSilverBrush}" BorderBrush="{StaticResource MediumSilverBrush}">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal"/>
<VisualState x:Name="Disabled">
<Storyboard>
<DoubleAnimation Duration="0" Storyboard.TargetName="PART_DisabledVisual" Storyboard.TargetProperty="Opacity" To="1"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Grid x:Name="PART_Root" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" Height="30">
<Grid.Resources>
<SolidColorBrush x:Key="DisabledBrush" Color="#A5FFFFFF"/>
<ControlTemplate x:Key="DropDownButtonTemplate" TargetType="{x:Type Button}">
<Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualStateGroup.Transitions>
<VisualTransition GeneratedDuration="0"/>
<VisualTransition GeneratedDuration="0:0:0.1" To="MouseOver"/>
<VisualTransition GeneratedDuration="0:0:0.1" To="Pressed"/>
</VisualStateGroup.Transitions>
<VisualState x:Name="Normal"/>
<VisualState x:Name="MouseOver">
<Storyboard>
<ColorAnimation Duration="0" Storyboard.TargetName="Background" Storyboard.TargetProperty="(Border.Background).(SolidColorBrush.Color)" To="#FF448DCA"/>
<ColorAnimationUsingKeyFrames BeginTime="0" Duration="00:00:00.001" Storyboard.TargetName="BackgroundGradient" Storyboard.TargetProperty="(Border.Background).(GradientBrush.GradientStops)[3].(GradientStop.Color)">
<SplineColorKeyFrame KeyTime="0" Value="#7FFFFFFF"/>
</ColorAnimationUsingKeyFrames>
<ColorAnimationUsingKeyFrames BeginTime="0" Duration="00:00:00.001" Storyboard.TargetName="BackgroundGradient" Storyboard.TargetProperty="(Border.Background).(GradientBrush.GradientStops)[2].(GradientStop.Color)">
<SplineColorKeyFrame KeyTime="0" Value="#CCFFFFFF"/>
</ColorAnimationUsingKeyFrames>
<ColorAnimationUsingKeyFrames BeginTime="0" Duration="00:00:00.001" Storyboard.TargetName="BackgroundGradient" Storyboard.TargetProperty="(Border.Background).(GradientBrush.GradientStops)[1].(GradientStop.Color)">
<SplineColorKeyFrame KeyTime="0" Value="#F2FFFFFF"/>
</ColorAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ColorAnimationUsingKeyFrames BeginTime="0" Duration="00:00:00.001" Storyboard.TargetName="Background" Storyboard.TargetProperty="(Border.Background).(SolidColorBrush.Color)">
<SplineColorKeyFrame KeyTime="0" Value="#FF448DCA"/>
</ColorAnimationUsingKeyFrames>
<DoubleAnimationUsingKeyFrames BeginTime="0" Duration="00:00:00.001" Storyboard.TargetName="Highlight" Storyboard.TargetProperty="(UIElement.Opacity)">
<SplineDoubleKeyFrame KeyTime="0" Value="1"/>
</DoubleAnimationUsingKeyFrames>
<ColorAnimationUsingKeyFrames BeginTime="0" Duration="00:00:00.001" Storyboard.TargetName="BackgroundGradient" Storyboard.TargetProperty="(Border.Background).(GradientBrush.GradientStops)[1].(GradientStop.Color)">
<SplineColorKeyFrame KeyTime="0" Value="#EAFFFFFF"/>
</ColorAnimationUsingKeyFrames>
<ColorAnimationUsingKeyFrames BeginTime="0" Duration="00:00:00.001" Storyboard.TargetName="BackgroundGradient" Storyboard.TargetProperty="(Border.Background).(GradientBrush.GradientStops)[2].(GradientStop.Color)">
<SplineColorKeyFrame KeyTime="0" Value="#C6FFFFFF"/>
</ColorAnimationUsingKeyFrames>
<ColorAnimationUsingKeyFrames BeginTime="0" Duration="00:00:00.001" Storyboard.TargetName="BackgroundGradient" Storyboard.TargetProperty="(Border.Background).(GradientBrush.GradientStops)[3].(GradientStop.Color)">
<SplineColorKeyFrame KeyTime="0" Value="#6BFFFFFF"/>
</ColorAnimationUsingKeyFrames>
<ColorAnimationUsingKeyFrames BeginTime="0" Duration="00:00:00.001" Storyboard.TargetName="BackgroundGradient" Storyboard.TargetProperty="(Border.Background).(GradientBrush.GradientStops)[0].(GradientStop.Color)">
<SplineColorKeyFrame KeyTime="0" Value="#F4FFFFFF"/>
</ColorAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled"/>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Grid Background="#11FFFFFF" FlowDirection="LeftToRight" HorizontalAlignment="Center" Height="18" Margin="0" VerticalAlignment="Center" Width="19">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="20*"/>
<ColumnDefinition Width="20*"/>
<ColumnDefinition Width="20*"/>
<ColumnDefinition Width="20*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="23*"/>
<RowDefinition Height="19*"/>
<RowDefinition Height="19*"/>
<RowDefinition Height="19*"/>
</Grid.RowDefinitions>
<Border x:Name="Highlight" BorderThickness="1" BorderBrush="#FF45D6FA" CornerRadius="0,0,1,1" Grid.ColumnSpan="4" Margin="-1" Opacity="0" Grid.RowSpan="4" Grid.Row="0"/>
<Border x:Name="Background" Background="#FF1F3B53" BorderThickness="1" BorderBrush="#FFFFFFFF" CornerRadius=".5" Grid.ColumnSpan="4" Margin="0,-1,0,0" Opacity="1" Grid.RowSpan="3" Grid.Row="1"/>
<Border x:Name="BackgroundGradient" BorderThickness="1" BorderBrush="#BF000000" CornerRadius=".5" Grid.ColumnSpan="4" Margin="0,-1,0,0" Opacity="1" Grid.RowSpan="3" Grid.Row="1">
<Border.Background>
<LinearGradientBrush EndPoint=".7,1" StartPoint=".7,0">
<GradientStop Color="#FFFFFFFF" Offset="0"/>
<GradientStop Color="#F9FFFFFF" Offset="0.375"/>
<GradientStop Color="#E5FFFFFF" Offset="0.625"/>
<GradientStop Color="#C6FFFFFF" Offset="1"/>
</LinearGradientBrush>
</Border.Background>
</Border>
<Rectangle Grid.ColumnSpan="4" Grid.RowSpan="1" StrokeThickness="1">
<Rectangle.Fill>
<LinearGradientBrush EndPoint="0.3,-1.1" StartPoint="0.46,1.6">
<GradientStop Color="#FF4084BD"/>
<GradientStop Color="#FFAFCFEA" Offset="1"/>
</LinearGradientBrush>
</Rectangle.Fill>
<Rectangle.Stroke>
<LinearGradientBrush EndPoint="0.48,-1" StartPoint="0.48,1.25">
<GradientStop Color="#FF494949"/>
<GradientStop Color="#FF9F9F9F" Offset="1"/>
</LinearGradientBrush>
</Rectangle.Stroke>
</Rectangle>
<Path Grid.ColumnSpan="4" Grid.Column="0" Data="M11.426758,8.4305077 L11.749023,8.4305077 L11.749023,16.331387 L10.674805,16.331387 L10.674805,10.299648 L9.0742188,11.298672 L9.0742188,10.294277 C9.4788408,10.090176 9.9094238,9.8090878 10.365967,9.4510155 C10.82251,9.0929432 11.176106,8.7527733 11.426758,8.4305077 z M14.65086,8.4305077 L18.566387,8.4305077 L18.566387,9.3435936 L15.671368,9.3435936 L15.671368,11.255703 C15.936341,11.058764 16.27293,10.960293 16.681133,10.960293 C17.411602,10.960293 17.969301,11.178717 18.354229,11.615566 C18.739157,12.052416 18.931622,12.673672 18.931622,13.479336 C18.931622,15.452317 18.052553,16.438808 16.294415,16.438808 C15.560365,16.438808 14.951641,16.234707 14.468243,15.826504 L14.881817,14.929531 C15.368796,15.326992 15.837872,15.525723 16.289043,15.525723 C17.298809,15.525723 17.803692,14.895514 17.803692,13.635098 C17.803692,12.460618 17.305971,11.873379 16.310528,11.873379 C15.83071,11.873379 15.399232,12.079271 15.016094,12.491055 L14.65086,12.238613 z" Fill="#FF2F2F2F" HorizontalAlignment="Center" Margin="4,3,4,3" RenderTransformOrigin="0.5,0.5" Grid.RowSpan="3" Grid.Row="1" Stretch="Fill" VerticalAlignment="Center"/>
<Ellipse Grid.ColumnSpan="4" Fill="#FFFFFFFF" HorizontalAlignment="Center" Height="3" StrokeThickness="0" VerticalAlignment="Center" Width="3"/>
<Border x:Name="DisabledVisual" BorderThickness="1" BorderBrush="#B2FFFFFF" CornerRadius="0,0,.5,.5" Grid.ColumnSpan="4" Opacity="0" Grid.RowSpan="4" Grid.Row="0"/>
</Grid>
</Grid>
</ControlTemplate>
</Grid.Resources>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<Viewbox Grid.Column="1" HorizontalAlignment="Left" Margin="3,0,3,0" Grid.Row="0" VerticalAlignment="Top">
<Button x:Name="PART_Button" Focusable="False" Foreground="{TemplateBinding Foreground}" Template="{StaticResource DropDownButtonTemplate}" />
</Viewbox>
<DatePickerTextBox x:Name="PART_TextBox" Grid.Column="0" Grid.Row="0" IsHitTestVisible="False" Focusable="False" HorizontalContentAlignment="Stretch" VerticalContentAlignment="Center"
Style="{StaticResource BaseTextBoxStyle}" Height="30" FontSize="20" FontFamily="{StaticResource Roboto}" Text="" Background="Transparent">
</DatePickerTextBox>
<Grid x:Name="PART_DisabledVisual" Grid.ColumnSpan="2" Grid.Column="0" IsHitTestVisible="False" Opacity="0" Grid.Row="0">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<Rectangle Grid.Column="0" Fill="#A5FFFFFF" RadiusX="1" RadiusY="1" Grid.Row="0"/>
<Rectangle Grid.Column="1" Fill="#A5FFFFFF" Height="18" Margin="3,0,3,0" RadiusX="1" RadiusY="1" Grid.Row="0" Width="19"/>
<Popup x:Name="PART_Popup" AllowsTransparency="True" Placement="Bottom" PlacementTarget="{Binding ElementName=PART_TextBox}" StaysOpen="False" PopupAnimation="Fade"/>
</Grid>
</Grid>
</Border>
<ControlTemplate.Triggers>
<DataTrigger Binding="{Binding Source={x:Static SystemParameters.HighContrast}}" Value="false">
<Setter Property="Foreground" TargetName="PART_TextBox" Value="{Binding Foreground, RelativeSource={RelativeSource TemplatedParent}}"/>
</DataTrigger>
</ControlTemplate.Triggers>