There is no such thing as too big a caliber for elk, but there is plenty of calibers too small for them. Anything that reliably puts the meat in the freezer, and I do not mean using a 223 and popping the critter in the neck. If your shot is off a little, well, you just wounded an animal, and showed a lack of respect towards hunting. Plenty of calibers will kill elk reliably. 45/70 with good loads will drop them at less then 100yard with authority. 375H&H will put them down at nominal hunting ranges.

The most important thing is to pick a gun you will shoot, and shoot alot, and accurately.