Hey Buddy!
Hopefully you didn't accept that answer from a few years ago. lol.
I ran into the same issue myself with MAMEUI and MESSUI on my 4K TV.
I first tried increasing the font size (same as you) and got the same result.
Then I tried this instead (as per my screenshot), and it worked!
Just right-click on the mame or mess exe. and set your DPI settings as per my screenshot. Restart mame or mess, and it should be fixed.
[ATTACHED IMAGE - CLICK FOR FULL SIZE]
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