Basically once an item is maximized you can see its actual size using the RadTileView.MaximizedItem property which holds the maximized item. for example you can try something like this:
sender, Telerik.Windows.RadRoutedEventArgs e)
RadTileView tileView = sender
height = (tileView.MaximizedItem
width = (tileView.MaximizedItem
However, I am not sure if this approach would be the best option for your scenario. This is why I wanted to ask you to elaborate on your scenario a bit more. That will help us better understand it and better guide you in its implementation.
the Telerik team
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