I currently have three. My stepmother's old crock pot, that is so old it doesn't have a removable crock. (Hard to clean, that way.) I have a round 5-quart that I picked up at a thrift store and a smaller one (2 or 3 quart) I got from dh's grandmother. We had a 6-quart "Smart crock," but we wore it out.
I use the SMALLER crock pot for things like dried beans, lentils and split pea soup. It softens them up more quickly. My husband said it's because of the higher surface ratio of the crock, since the heat comes through the crock. (Hubby is an engineer.) It takes forever for these things to soften properly in the larger crocks.
I use the larger one for whole chickens, turkey breast, roasts, etc. Meatloaf does well in there, too, especially with a rack. I wrap the loaf in cheesecloth to keep it together. With the chicken, afterward, I'll pour the liquid through a sieve and into a freezer container. Then I have frozen chicken broth ready for when I need it. It's especially tasty if I've cooked the chicken with onions, peppers and carrots.