Walker was officially measured at the combine, so we do know he is at least 6-4.5 and might've grown. The Spurs premeasured before the season and got 6-5. I can't say that those other players did not grow or what they're measured at. One thing about Lonnie is that it's hard to find pictures of him when he's not slouching or leaning. Especially for a guy who has considerable height in his neck, that can make him seem a variety of heights. In most pictures with Murray, Lonnie looks to be basically as tall, but next to Johnson, he is noticeably shorter. That would put him in the 6-5-in-shoes category (Murray is 6-4 and Keldon 6-6 and maybe growing?)
Still, Walker's standing reach is 8-3.5, which is the shortest of the young perimeter players of note. For perspective, Murray is first with 8-7.5, Johnson next with 8-7 and White next with 8-5.5. In that way, Walker seems noticeably smaller than his counter parts. In others like wing-span and girth, Walker is one of the bigger ones.
As far as hair goes, I have had some form of afro since I was in the fourth grade. People overestimated how much of my height came from my hair, often by three or four inches. People's heads aren't flat above their hair lines. Obviously we all know that, but people don't take into account that a round and uniform 'fro on the outside is not made up of hair the same distance from the skull. The hair on the top of the head is more compact/shorter. That's a long-winded way of saying Walker's height might be underestimated because of how tall his hair was. Obviously, it did add to his height, and more than my hair did for me, but he also has a few inches of head that is still there.