Editing Font Size of Main Menu in Manhattan Theme

I know that there are articles detailing this process for other themes, however, I have had little success doing this in Manhattan, a child of Genesis. I want to change the font size of my main menu. I have a custom font installed and have been able to change attributes of that menu such as ‘letter-spacing’ and ‘text-transform’. I have even been able to alter the size of the submenu. I cannot, however, increase the font-size of the main menu in my style.css. My website is here. Any help would be appreciated!