Thank you for your question.
You should set the maximum size of the popup that contains the submenu and this should be done for all items that have a submenu:
Hope this will help you. Feel free to write back if you have any further questions.
the Telerik team
Q3’10 SP1 of RadControls for WinForms is available for download
; also available is the Q1'11 Roadmap
for Telerik Windows Forms controls.