If you post your website url I can take a look and let you know what CSS rule needs to be added.
This is May from Shopify.
To clarify your question a little more, are you wanting to reduce the font size on your dropdown menu?
If this is what you are after, I did some testings on my test store and managed to reduce the font size by editing the theme code. After I made the change in the theme code, the dropdown menu looks like:
Here’s how I did it:
From your admin, you’ll want to go to ‘Online Store > Actions > Edit code’.
In your ‘Assets’ folder, select ‘theme.scss.liquid’ file.
Search for ‘site-nav__dropdown’ by pressing Command + F on Mac or Control + F on Windows computer.
Inside the bracket, add the code below:
You can change the pixel size depending on how big you want your font to be.
5. Click ‘Save’.
It is important to note that once you make changes to ‘theme.scss.liquid’, you cannot revert back to older versions. Therefore, we strongly recommend duplicating your theme before making changes to your code, to ensure that your customers aren’t affected if you see any unintended results! If you are not comfortable with making this change, I highly suggest reaching out to our Shopify Experts. Alternatively, since you are using a theme made by Shopify, our theme support may be able to look into making the change as well if your account is eligible!
If this isn't quite what you were after, let me know and we can continue with our support.