I think 8 inches... Jonah Falcon the owner of the world largest known penis.. has a length of 13,5 inches and a girth of 8 inches circa... and still look thick! I know also that the optimal ratio between length and girth for not having erection problems is 1,32.. i.e. for example 17 cm / 13 cm = 1,307.. is a longer trhan thicker penis but still not have problems in mantaining erection... an higher value is at risk of erection weakness