Canvas height & width bind to xml - wpf

Question

In the past I created a winform user control that inherit imagebox control, which the user can select part of the image by drawing a rectangle with the mouse.
Now I want to do the same with WPF. I wrote this XAML code:
<StackPanel Width="321" Height="241">
<Canvas x:Name="SampleImageCanvas"
MouseMove="SampleImageCanvas_MouseMove"
MouseDown="SampleImageCanvas_MouseDown"
MaxHeight="240" MaxWidth="320" >
<Canvas.Width>
<MultiBinding Converter="{StaticResource SizeConverter}" Mode="OneWay">
<Binding XPath="./Size/Width" />
<Binding XPath="./Size/Ratio" />
</MultiBinding>
</Canvas.Width>
<Canvas.Height>
<MultiBinding Converter="{StaticResource SizeConverter}" Mode="OneWay">
<Binding XPath="./Size/Height" />
<Binding XPath="./Size/Ratio" />
</MultiBinding>
</Canvas.Height>
<Canvas.Background>
<ImageBrush x:Name="SampleImage" ImageSource="{Binding XPath=./SampleImage}" Stretch="Uniform"/>
</Canvas.Background>
<Rectangle x:Name="ROI"
Canvas.Top="{Binding XPath=./ROI/Top, Mode=TwoWay}"
Width="{Binding XPath=./ROI/Width, Mode=TwoWay}"
Height="{Binding XPath=./ROI/Height, Mode=TwoWay}"
Canvas.Left="{Binding XPath=./ROI/Left, Mode=TwoWay}"
Stroke="#FFD13750"
/>
</Canvas>
</StackPanel>
The data is stored in an xml document. the image is bigger than the canvas, so it has to be stretch uniformly, to fit the max size of the canvas, and I want the canvas to resize to the size of the image scaled display, so the image fills 100% of the canvas with no margins.
when I'll have this, the mouse move/enter/exit event will be set only to the image borders.
The problem is the canvas height binded value, which cause the image not to be shown although it has beec defined same as the canvas width which works, and the convertor returns the corect value.
When I set the height to exact value instead of binded value, like Height="240", it works.
Thanks!