the popup class fits to the size of his content. so what you can do:
<Grid Name="MainGrid" Background="Green" SizeChanged="MainGrid_SizeChanged">
<Popup Name="mainpopup" IsOpen="True">
<Grid x:Name="FullSize">
<Border Name="MainBorder" Background="LightBlue" HorizontalAlignment="Center" VerticalAlignment="Center" Height="400" Width="1300">
</Border>
</Grid>
</Popup>
</Grid>
and in c#:
private void MainGrid_SizeChanged(object sender, SizeChangedEventArgs e)
{
FullSize.Width = MainGrid.ActualWidth;
FullSize.Height = MainGrid.ActualHeight;
}
Microsoft Certified Solutions Developer - Windows Store Apps Using C#