For my money, the actors face is fitting (and I think he'll do a fine job), but they need to replace the body with cgi so as to make him massive, not Hulk size mind you - but somewhere between that and normal human would have been nice.
On the one hand, getting someone like The Rock, say, to play Apocalypse might have been interesting from a basic build and voice perspective. I can definitely appreciate the disconnect between their choices and how Apocalypse is normally drawn in the comics. On the other hand, I'm thinking back to all the people who said Hugh Jackman was too tall to play a convincing Wolverine, and I'm willing to extend the benefit of the doubt on that score (I know some small minority of people are still upset about that, but frankly its obvious they matter exactly not at all).
I also remember Cameron's choice of Robert Patrick to play the T2. The obvious choice was to pick someone even larger than Arnold to play the T-1000, because that would then provide an obvious visual cue that this was a more powerful and menacing terminator. But by going the exact opposite direction, they actually managed to play the visual disconnect in a way that subtly conveyed the notion that the T-1000 was significantly more advanced than Arnold's T-800, because it had to be for that tiny frame to be throwing Arnold's much larger frame around.
Given that they do seem to be touching on Apocalypse's shape changing abilities in the movie (the previews apparently shows him at least changing size significantly) it is possible that his "normal" size being "not huge" might make it all the more dramatic when you see that his size is really anything he wants it to be.
