Megamenu font size

Hi
I’m using a “story” theme from Pexeto. This allows me to attach a Mega sub-menu to any main menu item.
The default is for the font size and style on the main menu to appear on this mega sub menu.
I would like to use the same font style, but, because of the amount of text on the sub-menu I want to reduce the size of the font on the sub-menu only.
As I don’t have a clue how to bring this about, I would appreciate any help.
Thanks