If you button the coat, lay it flat and pull the pits out to the max, you measure from edge of fabric just under the sleeve on one side to edge of fabric on the other, regardless of where the side seams fall and when you double it, it should be the coat size. If you got a 21" p2p, that roughly translates to a 42 which is usually in the 18 to 19" range.
I dunno. Maybe it's a slender cut?
Yeah, that's pretty much what I did. I'll double-check again tonight. I'll try it on again for the heck of it. I think the sleeves were too short otherwise it might be OK if I lost about ten pounds or so.