You can make them any reasonable size you want, but globally, all programs are affected:
Right-click the desktop in area clear of shortcuts, select: Properties>Appearance>Advanced.
In the "Item" drop-down box, select Menu
It will show the size of the Menu strip and color. Don't bother with this, many programs will not allow the color to be changed and the size is automatic unless you are making the strip size bigger.
Under that is the font styles to choose from and the font size.
It will automatically increase/decrease the size of the menu strip as you change font sizes.
Set as you wish, it's effects are shown instantly in the sample window.