First congratulate them for their templates.
I use the template sj_joomla3
Can you tell me where I can change the size of the main menu fonts. ?
Thank you.

Dear jortoa

We're sorry late, you can submit the ticket www.smartaddons.com/support/tickets to get our support faster

about your question
-> you can add the css below and change to your size at the end file /templates/sj_joomla3/css/template.css

ul.navi div.level1.separator, ul.navi a.level1 {
font-size: 18px !important;
}

then clear cache joomla to get the new css for your site

thanks