You need to go to your WordPress dashboard.
Settings
Multi-level Navigation plugin
Appearance
(Setting is where you can change what you have in your menu)
This is where you past the new CSS code, from the Multi-level Menu CSS navigation page.
https://geek.hellyer.kiwi/products/suckerfish_css/
Take the old code and save it on a word doc just in case you need it later. But delete it from the Appearance box and copy/paste the new code, then save it. Then you just have to change the font size and line height in new CSS code. (the one you just paste/copy into the Appearance box). I changed my font from 18px to 10px and the line height from 40px to 20px, and it fit my menu drop down just fine