The size thing is a bit overblown. He doesn't have ideal size for an NBA PF, but it's not as if that's the one thing--or even the main thing--holding him back.
Thomas has a 7'2" wingspan and 9'0" standing reach. There are numerous NBA PFs and PF/C-types with similar length who also lack elite athleticism, including...
David West: 7'4" wingspan, 9'0.5" standing reach
Draymond Green: 7'1.25" wingspan, 8'9" standing reach
Kevin Love: 6'11.25" wingspan, 8'10" standing reach
Greg Monroe: 7'2.25" wingspan, 9'0.5" standing reach
Paul Millsap: 7'1.5" wingspan, 8'9.5" standing reach
Carlos Boozer: 7'2.25" wingspan, 9'0.5" standing reach
Nick Collison: 7'1.5" wingspan, 9'0" standing reach
Carl Landry: 6'11" wingspan, 8'6.5" standing reach
Tyler Hansbrough: 6'11" wingspan, 8'10" standing reach
Kris Humphries: 7'0".5 wingspan, 8'10.5" standing reach
Matt Bonner: 6'8.75" wingspan, 8'9.5" standing reach
All of those guys are currently in the league and have had--or are on the way to having--long NBA careers. The difference between Thomas and those guys is that Thomas doesn't have one or more standout NBA skills.
And for all of the talk about his mid-range game, if you're an NBA GM that wants to find a guy who can potentially develop stretch 4 range, a player who shot 62% from the FT line in college (college FT shooting is a strong predictor of NBA 3P%) and attempted 20 threes in four years isn't going to be at the top of your list. Contrast that with someone like Romero Osby (who did get drafted a couple years ago, despite being even older than Thomas is now), who flashed a bit more potential as a shooter from the FT line (74% for his career) and 3-point line (39% over a small sample of 102 3PA).
Add in Thomas' age, and you're looking at an older prospect that lacks a standout NBA skill. There is no arguing that more length would help, but his size isn't as prohibitive as it's made out to be...and that's good news for him, because that's one thing he can't fix.