Viewbox and Canvas - wpf

Question

this is my first intervention here. I'm sorry in advance for my poor english and i wish i will be clear.
I'm Making a custom control. (i'm a beginner in wpf) his goal is depending of a Property "Type Control". It could be a simpe textblock displaying data (and change background color depending on level property), it could be a marquee text or could be a picture.The goal here is to make a dynamic control. I explain this but my real trouble his the marquee part.
This is my XAML code :
<ControlTemplate TargetType="{x:Type local:ControlPerso}">
<Border Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}" x:Name="PART_BORDER" VerticalAlignment="Stretch" CornerRadius="10" Height="Auto">
<Viewbox Margin="2" StretchDirection="DownOnly" x:Name="PART_VB" Stretch="Fill">
<Grid x:Name="PART_GRID" ShowGridLines="false" VerticalAlignment="Center" Height="Auto" >
<Grid.RowDefinitions>
<RowDefinition x:Name="PART_ROWLABEL"></RowDefinition>
<RowDefinition x:Name="PART_ROWSCROLL"></RowDefinition>
<RowDefinition x:Name="PART_ROWIMAGE"></RowDefinition>
</Grid.RowDefinitions>
<Grid Grid.Row="0" x:Name="RowLabel" VerticalAlignment="Center">
<TextBlock x:Name="PART_LABEL" Text="{Binding MyText, RelativeSource='{RelativeSource Mode=TemplatedParent}', UpdateSourceTrigger=PropertyChanged}" Background="Transparent"
FontFamily="Verdana" FontSize="{Binding FontSize, RelativeSource='{RelativeSource Mode=TemplatedParent}'}" FontWeight="ExtraBold" HorizontalAlignment="Center">
<TextBlock.BitmapEffect>
<DropShadowBitmapEffect Color="Black" Direction="320" ShadowDepth="2" Opacity=".9" Softness="0.3" />
</TextBlock.BitmapEffect>
</TextBlock>
</Grid>
<Grid Grid.Row="1" x:Name="RowScroll" VerticalAlignment="Center">
<Canvas x:Name="PART_ContentControl" ClipToBounds="True"
HorizontalAlignment="Stretch"
VerticalAlignment="Stretch"
Height="{Binding ActualHeight, RelativeSource={RelativeSource Mode=TemplatedParent}}"
Width="{Binding ActualWidth, RelativeSource={RelativeSource Mode=TemplatedParent}}" >
<Canvas.Triggers>
<EventTrigger RoutedEvent="Canvas.Loaded">
<BeginStoryboard>
<Storyboard x:Name="ContentTickerStoryboard" Storyboard.TargetProperty="(Canvas.Left)">
<DoubleAnimation Storyboard.TargetName="PART_Content" RepeatBehavior="Forever" />
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Canvas.Triggers>
<ContentPresenter x:Name="PART_Content"
HorizontalAlignment="{TemplateBinding ContentControl.HorizontalAlignment}"
VerticalAlignment="{TemplateBinding ContentControl.VerticalAlignment}"
Content="{Binding MyText, RelativeSource={RelativeSource Mode=TemplatedParent}, UpdateSourceTrigger=PropertyChanged}"
ContentTemplate="{TemplateBinding ContentControl.ContentTemplate}" TextBlock.FontSize="{Binding FontSize, RelativeSource='{RelativeSource Mode=TemplatedParent}'}">
<ContentPresenter.BitmapEffect>
<DropShadowBitmapEffect Color="Black" Direction="320" ShadowDepth="2" Opacity=".9" Softness="0.3" />
</ContentPresenter.BitmapEffect>
</ContentPresenter>
</Canvas>
</Grid>
<Grid Grid.Row="2" x:Name="RowImg" ShowGridLines="False" VerticalAlignment="Center" Height="Auto">
<Image Source="C:\Users\Zebody\Pictures\thk.jpg"/>
</Grid>
</Grid>
</Viewbox>
</Border>
</ControlTemplate>
My trouble : Viewbox works fine with the grid row's picture and grid's row "label" (resize the font perfectly). But it's not working with rowscroll (marquee part). The font never resize. It seems it's because the canvas. If i remove height and width, resizing works but canvas doesnot fill my custom control.
What i Whant to do : The canvas fill all my custom control and could resize the font when i resize my custom control (in a mainwindow).
Could anyone help me how to do ?
Thx in advance and sorry again for my poor english