Circular Batard
Here's something probably a thousand people on this site have figured out before me, but just in case someone hasn't thought of this yet:
The problem: A 12 inch diameter dutch oven. Handles boules quite nicely. Still, you don't always want the boule shape. The maximum size for a batard would be 12 inches of course. Unless you don't mind a circular batard, which allows you (pi X diameter) or 3.14 X 12 inches = 37.7 inches for your batard length, or at least external circumference. Now that's a substantial batard.