Aligning content in a WPF Viewbox

Try VerticalAlignment="Top" and HorizontalAlignment="Left" on your viewbox. It will cause it to be anchored to the top and left side.

<Grid>
    <Viewbox VerticalAlignment="Top" HorizontalAlignment="Left">
    ...
    </Viewbox>
</Grid>

If you want it to completely fill (but keep it uniform) you can use Stretch="UniformToFill"

According to MSDN the Viewbox is used to stretch the child elements. Since the child elements would be stretched, you would have to set the content alignment of the children.

You may want to look at this for more information on the Viewbox: How do I keep aspect ratio on scalable, scrollable content in WPF?