Shaq is taller than Walton. Here they are in 1998 when Walton was 45 years old.
Barefoot, Shaq was measured at 7'1" before the draft,
Walton supposedly was a 7 footer who preferred to be listed at 6'11" because he thought 7' sounded freakish. I'd give Kareem a solid 7'2", he was measured at 7'1 5/8" in college, but I'd guess that he grew to atleast a full 7'2".
David Robinson was shorter than O'Neal and supposedly measured at 7'0" barefoot for the 1988 Olympics. Probably a case of being listed at his height in shoes. Tim Duncan was measured at 6'11" barefoot before the draft camp.
Ewing was anywhere between 6'11"-7'0' IMO, but his wingspan was truly freakish, Olajuwon claimed to be 6'11", but I always heard he was really 6'10".
Kevin Garnett is definitely shorter than Shaq and just a hair taller than Duncan. Garnett reluctantly admits to be 6'11 3/4" and I believe that'd make him about 7'1" in shoes.
Artis Gilmore said Wilt was a 1/4" inch shorter than him so that puts Gilmore at 7'1" and change.
Yao supposedly grew a bit after his pre-draft measurement and ended up at 7'5 3/4" barefoot so basically 7'6".