Update: this morning I tried MC again on my iMac 21.5 retina and it seemed (briefly) that the above problem had cleared up. I Quit and reloaded MC a few times, and the font size stayed the same. I then went to .. Select Font to check the font. It was Helvetica 24. (Should've been 12, but hey, as long as it didn't keep doubling I'd be happy.) I clicked OK on the select font dialog box (though I didn't change anything). Quit MC. Rentered. Font had doubled in size. Quit and re-entered. Stayed the same. Checked the font size in .. Select Font. Had changed to 48. Quit and reentered. Doubled again. So it seems to me that somehow going to the ..Select Font dialog and clicking OK, whether you change the size or not, will cause the font size to double when you Quit and reenter MC.
So, the way I got the font size I wanted, and got it to persist was ... select font size 12. The font goes very small, probably to 6. Quit MC and renter. The font size is 12. Don't go anywhere near the .. Select Font dialog!