The important thing to note about Ernie Sims is his undersized problems are overblown and if you see him on the field you'll know why. He's 5'11" and 230+ pounds right now...he aims to be 240 pounds.
You want to see a big difference? Stack up a 6'3" 240 pound guy next to a 5'11" 240 pound guy and let me know which you see as being the more explosive powder keg of power. Sims is STOUT. He'll have no trouble being physical with blockers or crunching the crap out of runners...no more than Zach or Tedy Bruschi that's for sure.
The only area he'll have trouble, is in zone coverage I believe that his 5'11" frame will not be as physically imposing on the passing lanes as a 6'3" guy would be. Boomer has an excellent point though, at 5'11" Sims DOES have a 41 inch vertical...so his instincts and his ability to jump after the ball MAY make up for the lack of height. I think Sims would be great at the weak side spot in this D. But, who do you move to strong side? If you get Arrington for that, where do you put Crowder? Even worse, does drafting Sims mean you're finally cutting Zach loose?