I would say a big one as well. I have a 6l or maybe it's 6.5l and it's big enough to fit a decent sized lamb leg in for roasting (if the shank is cut and bent over). If it's not big enough to put a roast in it's not enough IMO.
I know that a couple of the girls have a big one and a small one but I don't really have enough money or space in my kitchen to justify going down that path.