To change the size of a menu you can use the control buttons and scaling on its frame like any other element.
The Menu has two additional sizing features:
1. Scale to Frame
When 'Scale to Frame' is checked, the menu will always adjust the size of its buttons to fit the frame.
When 'Scale to Frame' is un-checked the menu buttons will retain their size regardless of the frame (note that with Scale to Frame disabled you can't make the frame smaller that it content)
Spacing lets you adjust the spaces in between the menu buttons.