The capacity (in liters or quarts) isn't as important to know as the dimensions. What is the diameter? What is the depth? Capacity only tells you "how much" you can cook but tells you nothing about "what" you can cook.
I like a 10 inch or 12 inch DO personally. If you're only going to cook soups, stews, and similar one pot meals in it then it isn't as important. But if youu're going to bake in it (a good blackberry pie needs room to fit a 9 inch pie pan in with room to spare round the edges) then it DOES matter. For most cooking or baking the regular depth will do fine but it you're going to bake breads that rise, or taller meats (whole hens or small turkeys or taller roasts) then you'll need the deep sided ones.
I prefer the camp style with legs and a rim around the lid for a couple of reasons:
1. If you're baking (or even stewing) it needs a way to hold live coals or charcoal on the lid to maintain even heat all round like a conventional oven.
2. the lid can be inverted and placed on the coals in the morning (or whenever) and used as a griddle.
Here's a link to a series of good videos on the subject from Texas Parks and Wildlife:
www.youtube.com/watch?v=F2iG4aMKBCQ
