After a lot of testing I finally found the solution. Yessss
You have to go in "Change skin fonts". 1
Then "Modify Windows 7 / 8 part fonts..." 2
In my case it is necessary to modify the fonts "Section link - mouseover" and "Section link - normal".
So select the font to modify. 3 (one by one )
Then click on "Change font...". 4
Now you just have to change the size.
And to finish, Apply. (Be careful, you have to save or it won't work) 5