Hi Abdulhameed,
You need to modify the TopLevelHeaderTemplate. You can obtain that by generating the ItemContainerStyle in Expression Blend. In the TopLevelHeaderTemplate, change the width of the Grid that is contained in the PopUp element:
If you need to apply the same change for the deeper level of RadMenuItems, you also need to modify the SubMenuHeaderTemplate in the same way.
I hope this helps.
Greetings,
Dani
the Telerik team
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