It looks like you need a bit of help with the site overall.
To change the font size it is controlled by the Cascading Style Sheet (CSS). If you do not know what that is you can read about CSS here:
www.w3schools.com/css/
The file you need to change is:
femixsports-aura.fr/wp-content/themes/rt.../menu.css?ver=4.1.10
Specifically the rule:
.gf-menu.l1 > li > .item {
font-family: Arial Narrow, Arial, Helvetica, sans-serif;
font-size: 16px;
line-height: 23px;
padding-top: 7px;
padding-bottom: 7px;
position: relative;
text-transform: uppercase;
}
However this is not recommended, you should make a custom file which does not change the original files, only the changes you require. More about that here:
www.rockettheme.com/forum/free-joomla-st...-or-less-custom-file
So you would need to create a file called rt_oculus-custom.css and place it in "/templates/rt_oculus/css" in your folder structure.
Then you add the CSS you want to the file, so for example:
rt_oculus-custom.css would contain:
.gf-menu.l1 > li > .item {
font-family: Helvetica, 'Helvetica Neue', sans-serif;
font-size: 20px;
line-height: 23px;
padding-top: 7px;
padding-bottom: 7px;
position: relative;
text-transform: uppercase;
}
I can help with this if you like.