Depends on your beer. Some beers take a bit of grain, some take a bunch.
The hopper I put on my crusher seemed small, but I was in a hurry just to put somethign on and test the crush (it's a homemade crusher). It will handle just over 2 lbs.
I marked the 2 lb. line and use that to measure the grain I crush. As it is, I don't see the need to upgrade to a bigger one, myself. I can always stoop down and scoop up another 2 lbs. of grain easy enough.
I've seen them with 5 lb and larger hoppers though. Mine is roughly 4x6x10 inches, but that is an estimation, as I am at work and the crusher is at home in the garage.