Dear @ninaverena
I understand that sometimes access to the website is not possible :)
I hope i can run you through and you apply this to your case:
For this we will need help of the Inspection tools provided within Google Chrome's browser:
https://www.khanacademy.org/computing/computer-programming/html-css/web-development-tools/a/using-the-browser-developer-tools
along with adding our own Custom CSS within "Lay options- custom css & html" :

When we right click on your webpage we can choose "inspect" from the menu:

So clicking on the menu which you wish to change:

Here on the left we have some related data, that we will use to target the menu with CSS e.g:
.menu-item-171
a
[data-id="10"]
span
I changed my menu item's font size by targeting it like this:
.menu-item-171 a {
font-size:48px;
}
Because this is a link <a> i have added the 'a' to be sure that the change occurs, you could also target it further to be sure by adding 'span' to the code.
I hope this help's you - target the Menu-item area or the individual menu-items this is your choice - then use 'font-size:20px;' ( change the 20 px to your preferred size ) - adding the code to Custom CSS
Let me know how you go! :)
Best wishes and have a wonderful day
Sincerely
Richard