I think it depends on the height and condition! if guy is 5' 4'' and has a cold 15 inch arm with low fat, it can look good, but as someone gets taller, not quite as good, mayb on a scale of around 1/2 inch one would need extra or just under for every inch taller they are. Also the lenght of the arm plays a part, and the peak of the bicep, mine are quite flatish.
Also some arms look much better pumped, some can get up to 1 inch extra, Franco Columbu could, and they looked very large, considering even in contest shape his arms were only around 17.5 inches.
Mine are 15.7 cold with low fat at 5' 8'' tall, has allway been my weakest body part btw.