The only person who would be to big to play as would be Devastator. Sentinel would make a great Decepticon Commander though. If a DLC is ever made.
Highly doubt he'd be too large. IIRC, he was maybe a foot or two taller than Optimus. And when it comes to 30' tall robots, that's not that much.
I wouldn't hold you breath, but don't give up either. Let's all remember that the ROTF DLC took awhile to get made.
I don't have the DOTM game, so I don't really care too much what they do, but it would be cool for those that have the game to be able to play as Sentinel Prime. It might not fit into canon too well, but then neither does Jazz being in the ROTF game.