Trust me bro, I am natural with 21" arms. Its just that my camera explodes each time I try to photograph them.
Joking aside, 20" nattie arms would be very difficult, whilst staying lean. If anybody looks beyond the bloated Weider mags measurements, you would see that Arnold taped at 19.75 close to the 72 Olympia, with Sergio Olivia having the biggest lean biceps around that time (just over 20, IIRC). Neither Arnold nor Sergio were natties. Both were genetic marvels, great champions who were, however, not naturals.
A bulked up nattie with close to 20" guns around 20% BF would look darn impressive.
A reasonably lean nattie with 17" guns would look very impressive too.
Here's shots of Reg Parks, Steve Reeves, Bill Davey (former Musclemania HW champ) and Ade Rai (another former HW musclemania champ who claimed to be natural). All of them are 6'0" or taller. None of them have arms over 18" (or weigh over 230 lbs - Rai at 6'0" 205 lbs is the smallest - but his amazing genetics probably creates the best illusion).