You can change the font size on line 2076 in your style.css file from your child theme –
.genesis-nav-menu {
clear: both;
color: #ccc;
font-size: 13px;
font-weight: 600;
letter-spacing: 1px;
line-height: 1.5;
margin: 0;
padding: 0;
width: 100%;
}
You can find this section by doing Command + F on your keyboard if you’re using a Mac, and Control + F if you’re using a PC. Just search for genesis-nav-menu
and you’ll find the section it’s located in.