I see that you cannot directly control the filters font size for mobile. but what i would suggest is to take advantage of "text formats".
Create a Text format that you wish to apply to the Thumbnail Grid Filter. Each Text format created will have a 'custom class'
Here I have the text format 'Acme' with HTML class "_acme".
Appearance > Customize > project thumbnails > Thumbnail Grid Filter >
Apply your new format:
We can target this HTML Class / Text Format in:
Lay Options > Custom CSS & HTML > Custom CSS for Mobile Version
This way we maintain the mobile breakpoints that are applied to the whole website.
Some code like this can be added to address the font-size of the Text Format, it could be 'px' or '%' or any other CSS UNIT
Hope this has helped Willie, have a wonderful day and good luck! 🌝