IF a guy has successful surgery (and a good many don't!) he has an increase in flaccid size. that usually becomes less noticeable when erect. Doctors post results like 1.5" increase in flaccid length, but only 0.5 inch increase in erect length. It seems to be more about locker room pride thing, than actually a signifigantly larger functioning sexual organ.
That increase is not necessarily permanent, it depends on using a stretching device post operation during the healing process, otherwise some reported SHORTER penis length results. Needless to say, results vary.
Dr Robert Stubbs in Toronto said in an interview that most of the guys who see him are not smaller than average. Even above average guys seem insecure and want more than nature gave them.
It hardly seems worth the risk of damage to sensation and enjoyment and so on, but psychological issues seem to take precedence for some people.