There are no idicators of penis size, besides looking at the penis.
I haven't seen any penises in real life, beside the random lockeroom penis, and those are all soft so i can't say how big they are hard.
I am arabic, and I'm 7 long, 5 around. I've never heard of jelqing until the internet (I honestly think it being pegged to Arabs discovering it is a load of bullshit).
if anything, arab penis size is probably diverse, because the middle east was the central trade route between east and west for more than 2000 years. So if there was any genetic factors in penis size, arabs would probably be the most diverse in penis size.