This comes up a lot, and you'll get more opinions if you do some searches. However, as far as smoking just one, as already mentioned, you can squeeze a pretty good size one between the two handles on the top grate. I believe my largest was either a long-ish 14-pounder, or maybe a fifteen pounder, and most of my packers have been smoked like this with pretty good luck. I suggest laying some foil under the end of the flat around where the handle is to help protect the end from the higher temps there.
As for smoking a larger one on the bottom rack, I'd opt for an idea that I found in an older thread. The poster (Keri C.?) suggested cutting a large enough size chunk off the point end to make the brisket fit, laying the chunk beside the brisket. (By the way, Adam Perry Lang suggests doing the same in his new book.) Anyway, Kerry pointed out that since the point has so much more fat to render, it wouldn't overcook before the brisket was tender in the flat, and I've done this before and will confirm that it's a fine option. As on the top grate, I'd lay some foil under the flat end of the brisket where it lays past the edge of the pan, and maybe even the other end, since it's the bottom grate.
