Historically, of course, NBA PF's (think blake griffin) are big and powerful -- 6'9 - 6'10" 250+. That's changing with small ball. These days, some smaller "PF's" can play like 3's on offense and then defend NBA bigs down low (Draymon Green). Draymon's only 6'7-8 230. That's about the weight I expect Lydon to play at (+20 lbs. for a 19 y/0 isn't that big a stretch), and Lydon's 6'9. At that height and 225-230, I think he could play a small ball PF at the next level and be a good secondary rim protector. At SF, he'll need to show some quickness and ball handling. We know he can light it up from deep. If he develops a serviceable dribble-drive game (or shows potential), at 6'9 I think he could be a matchup problem for NBA 3's. Right now, he's got length but not strength. That will come ... probably +10 lbs. this year.
Enjoy him while you can.