We are aware of the issues that come up with setting Margins to control the RadTileViewItems height/weight, since those values are applied at all times despite the item's state - Restored, |Maximized, Minimized. And the workarounds for them might be really complicated, since you will have to change the Margins depanding on the Item's state or other proprieties. Therefore it is better to wait for the custom size feature that we are working on and that will be released with the Q1 2011 Beta release scheduled for the mid of January. You can track this feature's progress here
I hope this time frame is acceptable for you.
the Telerik team
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to help you get started with RadControls for Silverlight