Hello,
1) You can change the font size for the main menu by adding this to the custom.css
#s5_nav {
font-size: 0.8em;
}
The font type is based on your Highlight Font selected in the Template Manager.
2) There are several font sizes coded throughout so there isn't a single button to change everything. You can try using the Font Size override option built into the Layout Tab of the Template Manager.
Regards,