<UserControl .....     Width="auto" Height="auto" >    
<Grid x:Name="LayoutRoot" Background="Transparent"  >        
<Popup Name="popUp" Height="auto" Width="auto" MouseLeftButtonUp="Popup_MouseLeftButtonUp" MouseMove="Popup_MouseMove" Opened="Popup_Opened" >            
<Border Name="bounds" Background="White" Height="auto" Width="auto" Opacity="0.3">
</Border>        
</Popup>    
</Grid>
</UserControl>

bounds.Child = SetUIElement(); // Устанавливает некоторый UIElement

private void Popup_Opened(object sender, EventArgs e)
{
//оба размера равны 0. Вот собственно в этом и проблема...            
visualContentWidth = popUp.ActualWidth;            
visualContentHeight = popUp.ActualHeight;        
}