It depends from the menu type, you have chosen: if you have a splitmenu, you go to “templatesja_larixja_splitmenu” and open the ja-splitmenu.css, then you search the line in the part “TOP-NAVIGATION” with “font-size: 135%” and change the percent number.
If you have a transmenu, you go to the “templatesja_larixja_transmenu” and open the ja-transmenuh.css, then you search the line in the part “Added classes by Khanh …” with “font-size: 135%” and change the percent number as well.
If you are using a cssmenu (son of suckerfish), you go to “templatesja_larixja_cssmenu” and open the ja-sosdmenu.css, then you search the line in the part “STYLING THE MENU” with “font-size: 135%” and change the percent number as well.