Kozlov wont be a tall player, if he grows to be 1.80 it might not be tall enought in modern tennis but he seems more destined to be around 1.75 or something. He is not behind hitting puberty like some videos of him 13-years old has shown but he is late in the growing spur.
His game might look like Tomic or Young, lacks aggressiveness to dominate. I dont know how far the kid can go, certanly there will be open room for anyone trying to break throught in 4-5 years but I dont think he has it in him to dominate the tour, at the very best something like Hewitt is his fullfilled potential.
On the first point, you're hilariously misinformed.
If you look at the pics from this week of him standing next to other players, he's probably already near 1.75. He is shaking hands with Daniel Nguyen at the net, who is listed at 5'10 and is probably about 1.78. And Stefan looks taller than him. And he is only fourteen, so the question is not will he be undersized but how tall will he end up?
On the second point, the idea that you need to be a Federer like offensive juggernaut to be a great player is silly. You just have be really damn good overall. On this, it's obviously too early to comment about him. Having said that, I cannot think of a worse comparison than Hewitt. It's clear they are nothing alike as players.