4 Quart Crockpot-
A 4 Quart Crockpot will make enough food to feed 4-5 people. This size is perfect for a couple or a small family.
6 Quart Crockpot-
The 6 Quart Crockpot is the most popular size crock. Most recipes are designed using this size. A 6 Quart Crockpot will make enough food to feed 6-7 people. Most of the time when you are cooking in your crockpot you will want to have guests over, or make enough for leftovers, so this size is great for a family of 4 or 5.
7 Quart Crockpot-
The 7 Quart Crockpot will make enough food for 9 people. This size is great for cooking for cookouts, or for bringing food to church events or other gatherings.
16 oz Crockpot-
The 16 oz Crockpot brand slowcooker is called the "Little Dipper". The 16 oz crockpot is ideal for making dips for parties. You could also use it to make breakfast, to make lunch for the kids, or to make baby food.
They make crockpots in pretty much every size from 3 Qt to 7 Qt that you can think of, plus the small Little Dipper-like sizes, but the 4 Quart, 6 Quart, 7 Quart and 16 oz sizes are the most common.
Some crockpots work better, and have better features for the money so you might want to look at Amazon's Highly Rated Crockpots.