Thank you for your question.
To resolve your issue you have 2 options:
1. You can create a scrollable menu following the steps in this help article: http://www.telerik.com/help/silverlight/radmenu-how-to-create-scrollable-menus.html
2. Or you could create a multicolumn menu as illustrated in this help article: http://www.telerik.com/help/wpf/radmenu-how-to-create-multicolumn-menu.html
Hope this information helps. If you have additional questions please let us know.
the Telerik team
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