As for sizing, well I go by 'A3/A4 etc.' just to make it easier to word, but have the exact measurements on my website. Basically in an advertised 'A3', the portrait itself will measure 15x12 inches, or 37x30cm.
I am currently working on my next commission, which is an A2 graphite piece of a horse's head and partial body. Will be keeping me busy for a while I think!