you need to change the height and the width of the UserControl to Auto
and set the HorizontalAlignment and VerticalAlignment of your LayoutRoot element to Stretch
like this:
<UserControl x:Class="DataGridTest1.page2"
xmlns="http://schemas.microsoft.com/client/2007"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Width="300" Height="300" >
<Grid x:Name="LayoutRoot"
Background="Red" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
…
</Grid>
</UserControl>
Mohammad Shahrouri.