Honestly, I like having a bigger one, and I just cook for DH and myself. For one thing, I can put a whole chicken in it. Also, most crock pot recipes freeze well. I put the leftovers in serving-size portions in freezer bags.
You can't fill a crock pot all the way up to the top. I think the max is around 2/3, but you can check for sure in your book. It's nice to be able to cook a whole roast with lots of potatoes, carrots, and onions. If you can't eat it all right away, the leftovers will keep wonderfully as long as you eat them within about 4 weeks and store them correctly.