The C1GridExtender control supports column resizing on the grid at run time. To allow users to resize the grid's columns at run time, simply set the AllowColSizing to True. For example, with the following markup:
<cc1:C1GridExtender runat="server" ID="GridExtender1" TargetControlID="demo" AllowColSizing="true">
At run time, click and drag the right or left edge of a column to resize it.