In my opinion you can't predict.
I have a close bud who is quite long and very impressive hard. Under his jeans or slacks it looks like there's nothing there, but you'd be astounded when he drops his pants and starts getting hard.
I have another bud who's about 8 hard. Not a shower at all, gets pretty small when soft, but has a long flopper when hard. He showed me a picture of a friend of his who is 5'5 and has a thick 9 inch cock. Unbelievable!
Body size will make you think all kinds of things but can never be depended on.
I'm about 6'2, 175 lbs., have 6 inches hard and am 5 & 3/4" thick when hard. Everybody tells me how (thick) I am but under shorts or slacks you'd never know.