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.
If your takin' your girlfriend out tonight
You better park the car well out of sight
'Cos if they catch you in the back seat trying to pick her locks
They're gonna send you back to mother in a cardboard box !