@toolhead Glad to see you are thinking about a DO. They are a ton of fun and very rewarding to cook with. The more you use them, the better they get. I will give you my $.02 - I have ovens ranging from 8" to 16". Here is a shot of a 10" (4 quarts) and a 12" (6 quarts). Both of them are approximately 3.75" deep and both will fit on a weber cooking grate with the lid on.
If I were only to get one DO, it would be a 12". It will hold 50% more food, but more importantly, it is a good all around size to feed two or twelve. It is the one oven I use most. They also make a 12" deep oven which holds 8 quarts.
The other thing about the 12" is that it really works well with your grill. If you get a gourmet grate, you can do cool things like this:
Let us know what you end up with. This is a pretty good page to discuss the various sizes:
http://www.dutchovendude.com/cooking-select-dutch-oven.shtmlWinz