you can add this at the bottom of your template.css file:body .menutop li.root > .item {font-size: 1.5em;}
If you are using Gantry 4 (or later) you can add it to the bottom of your rt_TEMPLATENAME-custom.css file (e.g., rt_kirigami-custom.css).
Also, if you do not like the font type you can change that in the style tab of the Zephyr template manager