Someone else has got to find this annoying.
If you set the width property of a RadButton you can not see the button in design view in VS 2012.
As described in Telerik's PITs system.
If the Width property of a RadButton is set, its appearance in the Visual Studio 2012 Design view will be broken.The icons sprite starts showing when the width is sufficiently large, test with different values, e.g. 80px, 100px, 200px, 400px, etc.
This was reported over a year ago and only 3 users have voted to have it fixed. It has got to be effecting more users. Apparently it is not of high enough priority to get fixed.
If you are having this issue please vote for this here to get this corrected