Paladin, Dave makes good points. A lot of the big people I've trained with think someone has to be big to teach them anything, or be worth training with. FWIW, most of these people I have to really hold back with when training, because they are usually inflexible. (I'm 5'6", and heavy at 148 or so.)
Much as I hate to say it, something like TKD might actually be of utility to someone like Shaq. Anything that has longer-ranged strikes and kicks. Boxing might also be a good choice. People this big really need to work on speed and agility, because power obviously comes easy. For li'l guys like me, OTOH, speed and agility come easy. I work on power most of the time.
I guess I'm not really answering your question. Yes, I think big guys can be agile. I have seen a 300+ lb fat man whip through rolls while having wrist locks applied by about the closest the US has to a "master" martial artist. Can they be as agile as a little guy? Maybe not, but the big ones can make up the difference in distance and power. 'Kay?