It's too small if you can't fit your bread in ;)
For most of our bread we use a Lodge combo cooker thing because burning yourself on a hot dutch oven sucks. It'll just about fit a 900g loaf. It's completely useless for cooking normal things though unless you sand back the rough surface and re-season it.
I'd guess around 5-7L or 25-30cm is a good size for a normal cast iron or earthenware casserole.