I do think his size keeps him from being a top 5 pick, which is where some like to slot him. That said, he's the same height as Stanley Johnson, and he absolutely has the size and length to be a 3 in the NBA. The with/without shoes is a little silly at times, but you have to remember that listed heights for NBA players in media guides and such are always with shoes, so that's the point of comparison when looking at players he'll match up against in the pros. Still, LeBron James he is not.
He's a little smaller than James Young, and he really shouldn't be confused for someone who will play the 3/4. Also, he's only 19 and 2 months, which means it wouldn't be unheard of if he grew another inch. Furthermore, his standing reach was as tall as any pure SF in the draft (if you think Dekker is more of a 3/4 like I do). When he puts his arms up, he's plenty tall. Not to mention, he's pure muscle. Consider he's 3 pounds heavier than Dekker, who has three inches on him, and yet has a body fat percentage of only 5.3%. Again, he's young, and it will be hard for him to be impactful in year 1, but in the long run, he has plenty of size to be a starting 3 in this league.
But if you want to draft a player based on size for his position, George Lucas (DePaula) is your man. He's pretty much the same size as Winslow (6'4.5" w/o shoes), with a 7 foot wingspan, and center-like hands. And is a PG. And only turned 19 this past Sunday.