What size string for a string trimmer?

Somewhere I got the idea that each string trimmer was designed for a
certain diameterr, but I have two on which I can find no
recommendation. Maybe on one it was scuffed off when it was hit on
the ground to feed more string, but I don't think so.

Is there a problem using string too thin? Or too thick? How do I
know what to use?

Thanks.

(I'm either planning for next year or late for last year.)