The font size for the nav is actually coming from line 1988 in your css –
.genesis-nav-menu {
color: #ccc;
font-size: 13px;
font-size: 0.8125rem;
font-weight: 700;
letter-spacing: 1px;
letter-spacing: 0.1rem;
line-height: 1.5;
}Make sure and change the rem in addition to the px. You can use this site – https://offroadcode.com/prototypes/rem-calculator/ to find the equivalent size in px. ….or you can remove the rem size if you don’t want to use it.
That code above will change the size of the nav for all of your menus. If thats all your planning to have in the header area then that should be all you need.