Jack isnt idea size but he would be a monster in a 3-4... This isn't a Paul Dawson running around blockers, getting destoryed when they get their hands on him. The dude fits up where he's supposed to and he destroys lineman. I posted these in another thread but maybe you missed them...
As for Jaylon, I don't have an arsenal of his gifs but he has a pretty massive frame. 6-3 235 and he looks like he has 34+ inch long arms, and he's fairly stout at the point of attack..He does rely on his speed/instincts to beat blockers to the spot to make tackles, but he's so great at it he will most likely be able to do it in the NFL. His sidline to side line speed is just downright insane, Proably on par with Luke Kuechlys if not even better. I would take him in a heartbeat, and i'd come up with a scheme designed to let him run and round and make tackles.