All we're all doing is trying to predict if and how his game will translate to the NHL. I'm sure we've all predicted and will continue to predict both wrongly and rightly. Having made that disclosure, lol:
I can't see his game translating to the NHL. Skilled and decent IQ, that seems a given. But very slow skater, seems lazy at times. Smallish for a center. And doesn't seem to have a high compete/intensity level. The fire, per se. And I feel that is the missing element he would need to overcome the physical shortcomings and lack of speed. But hey, I could be dead-wrong. For example, Zach Benson is also a small center with high skill and IQ who lacks speed. But no one is more tenacious or works as hard in all 3 zones and has that all-out, do anything to win drive.
I'm not going to argue that Smith's game is going to translate - jury still out for me. But these reasons don't square.
Size: He's 18 years old and can't yet grow a beard. Very likely he has a little height left to grow and definitely has weight to add. He's 6'0" where average centers are just under 6'1". He's probably 185, so skinny, but again he will be gaining muscle until he's 22-23 at least. Size isn't the reason.
Speed: he has quick bursts and he has shown decent top end speed even as recently as WJC. issue is it's inconsistent. That's either a style problem (too passive and waiting for the play to develop, or not enough compete/intensity), an endurance problem (often looks clearly slower at mid/end of shift), or a burst problem (not high end enough fast twitch).
All of those can improve, but how much will they improve? It's simplistic to say he's "a slow skater" when he has shown anything but, but only at times.
On compete, he certainly comes across in all interviews as hyper competitive, but his style on the ice is less "dog on a bone" like Leonard and more "sit back like the hawk and wait." That may need to change or he may just need to be slightly more tenacious at times.
Is he going to be able to adapt his style and learn quickly? There's still a lot of time. But the variability of performance is more concerning to me than "size or speed."