If you are boiling or frying, you can hang the stove-top style Dutch Oven from a tripod over a fire or use on a burner. I've also used el Cheapo welded wire flower pot holders as pot stands for my flat bottoms and fry pans as well, over wood fire of course.

The 12" diameter are arguably the most common size used these days. The standard 6 quart is preferred for baking, the "deep" 8 quart for roasting and casseroles, stews, or other higher capacity recipes. I have more deep ones than standard size, but I have adapted my baking to accomodate the greater volume.