Hi Guillaume,
I don't think there is any way to change the height of the menu bar. CVI menu bars are inherited from the OS like most applications. So it will use the system wide settings and it will look like any other menu bar in the system.
You may to look into a a toolbar to have more control on how things look.
Here is a good example on how the FP works.
I hope this helps.
Regards,
Juan Carlos
N.I.