He's 6'8 6'9 but plays way bigger than his size.
Does he play way bigger than his height, though? I watched Sullinger in quite a few games and he hardly ever got above the rim. He did all of his offensive work with hooks, short jump shots, and post moves leading to layups. He seldom dunked the ball. Yes, he was double and triple teamed, and yes he worked hard on the glass...clearly a force to be reckoned with in college, but I worry a lot about it transfering to the NBA.
To me, it isn't that Sullinger is undersized, or even really un-athletic...he has good foot work and is pretty nimble in the paint...it's his lack of lift and his lack of speed (both laterally and straight line) that really trouble me. He is going to be as ground bound as Larry Bird was (and about the same height) but without Larry's shot, speed, or ball handling. He is going to plod up and down the court like Perkins and he is going to struggle to play the C's defense. He'll be too slow on rotations and he's never going to be able to show and recover on pick and rolls.
Still, if Danny picks him, I'll hope I'm wrong and he's right. IIRC, Sullinger wasn't nearly as ground-bound as a Freshman as he was as a Sophomore. Maybe, just maybe, he's had "the back" all year. A bad back will certainly slow you down and hurt your lift...a lot.
Ryan Hollins fits your description of a tall, athletic big.
A tall, athletic big with very poor skills, almost no fundamentals and minimal BBIQ. Still, he was playing passable team defense for the C's within a few weeks of being picked up just off energy and effort.