The column with the Icon is with Width="Auto". But like I said the problem is that Image control do not report its size until it is shown. So right now setting the image size manually is the best option. This way the column will resize.
Exposing property for the column that holds the Icon couples the control with the template too much. If someone modify the control template and forget to bind it, this property become useless.
I hope that the next version of Silverlight won't have this limitations regarding Image control.
the Telerik team
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