This is where you will need to use a plugin for editing/creating custom CSS or the child theme. The font and sizing for the main menu is found in the style.css around line 2199
.main-navigation li {
list-style:none;
background-image:none;
margin-top: 20px;
padding-left:0;
font-family: ubuntu-light,abel,sans-serif;
font-size: 1em;
line-height: 1.42857143;
}