Hello Sean,
To change the font size of the main menu and submenus in the “Expert Consultant Pro” theme, add the given CSS into your Customizer >> Additional Css and Publish it. You may change the font size as you want.

1. Font size CSS for Main Menus:

.navigation .mainmenu>li>a {
display: block;
font-size: 18px;
}

2. Font size CSS for Submenus:

.header.site-header ul.sub-menu li a {
font-size: 16px;
}

Thank you.