I was trying to edit submenu font size in Ubermenu but found no related section in Ubermenu interface and I solved it with this code custom CSS code.
.ubermenu .ubermenu-submenu .ubermenu-item .ubermenu-target {
font-size:16px;
}
Just go to Appearance>Ubermenu>General Settings>Custom CSS>Custom CSS Tweaks and paste it. Adjust the font size and there you go!
I spent hours trying to figure this out, hope it will help others to save some time 