We could stare at phone camera images taken at various skewed perspectives and try to compare his height to people whose heights we are even less sure about ...
Or ... we could just accept the actual measurements that we have so far.
http://www.draftexpress.com/profile/Markelle-Fultz-90302/Fultz' DraftExpress.com profile has measurements from various events last year (most all from April through August of 2016) that all put his height between 6' 3.5" - 6' 4.75" w/shoes. Only one event had him below 6' 4" and that was a USA Basketball event that had some other odd measurements on other players. The Nike Hoop Summit measurements tend to be pretty reliable and consistent with the way the NBA Draft Combine measurements are done.
At any rate it's probably a reasonable statement to say that he was a solid 6' 4" last summer.
He was still a young 18 then. i doubt that he has shrunk since then. He definitely looks like he's at least added quite a bit of weight since then. In games late this last season he looked to me like he was close to around 200 lb.