fluid layout in wpf is rendering enormous controls - wpf

Question

EDIT
Having removed the <ViewBox Stretch="UniformToFill"> from the xaml, it renders better - the size of the controls are more appropriate, however, as the Frame is empty when I'm loading the window, the whole app appears as a band across the center of my screen.
When I do load content into the frame, the rest of the window fits width to the frame, rather than to the screen resulting in empty space on the left and right of the Frame's content.
ORIGINAL QUESTION
I've never done a fluid layout before so what I've done is based entirely on what I've gleaned from answers to other questions on SO and around the interwebz.
So the following XAML, I figured, should be "responsive" which is to say that it would render much the same on every screen resolution without using fixed widths.
Unfortunately, the controls are rendering at enormous scale. What should be a 30x30 button looks to be rendering around 10x that size (see the screenshots below - the second one is the app maximized to fullscreen).
How can I change my xaml to make these controls render to the row heights and to the width of the display?
<Grid>
<Viewbox Stretch="UniformToFill">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="30"/>
<RowDefinition Height="50"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="60"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"></ColumnDefinition>
</Grid.ColumnDefinitions>
<!-- Titlebar -->
<StackPanel Grid.Row="0" Grid.Column="0" Orientation="Horizontal" Background="#2d2d30">
<StackPanel Name="spTitlebar1" FlowDirection="LeftToRight" Orientation="Horizontal" MouseDown="spTitlebar_MouseDown">
<Label Name="lblTitle" HorizontalAlignment="Left" BorderThickness="0" Foreground="White">IHS Towers SCADA</Label>
</StackPanel>
<StackPanel Name="spTitleBar2" FlowDirection="RightToLeft" Orientation="Horizontal" MouseDown="spTitlebar_MouseDown">
<Button Name="btnClose" Content="X" Click="btnClose_Click" Style="{StaticResource TitleButton}" />
<Button Name="btnMinimize" Content="_" Click="btnMinimize_Click" Style="{StaticResource TitleButton}" />
<Border BorderThickness="1,0,0,0" BorderBrush="#686868">
<Button Name="btnSettings" Click="btnSettings_Click" Style="{StaticResource TitleButton}" ToolTip="Settings">
<Image Source="Images/settings-icon.png" Height="20" Width="20"/>
</Button>
</Border>
<Button Name="btnReports" Click="btnReports_Click" Style="{StaticResource TitleButton}" ToolTip="View reports">
<Image Source="Images/reports-icon.png" Height="20" Width="20"/>
</Button>
</StackPanel>
</StackPanel>
<!-- Towers Bar across the top Height="50" -->
<StackPanel Grid.Row="1" Grid.Column="0" Name="spTowers" Background="#2d2d30" VerticalAlignment="Top" Orientation="Horizontal" HorizontalAlignment="Center" Margin="0,0,0,0">
<Border BorderThickness="5,0,0,0" BorderBrush="#686868" VerticalAlignment="Top"></Border>
</StackPanel>
<!-- Alert Window in the center -->
<Frame Name="frmBody" Grid.Row="2" Grid.Column="0" Margin="0" Background="#1e1e1e" NavigationUIVisibility="Hidden" />
<!-- Actions Bar across the bottom Height="60" -->
<StackPanel Grid.Row="3" Grid.Column="0" Name="spActions" Height="60" Background="#2d2d30" VerticalAlignment="Top" FlowDirection="RightToLeft" Orientation="Horizontal">
<Button Name="btnSMS" Click="btnSMS_Click" Height="30" Width="66" Margin="10,10,10,10" Style="{StaticResource RoundedButtonGreen}">Send SMS</Button>
</StackPanel>
</Grid>
</Viewbox>
</Grid>