Weber Smoky Mountain Questions
It depends on how you prefer to smoke it. A packer is made up of two distinct mussels and if you do as some suggest, seperate them prior to smoking then a very large brisket will fit on the 18". If you don't seperate them (the competition guys do according to the smoking class I took at the Midwest BBQ Institute in Kansas City) then it's going to depend on the shape to some extent as some are very flat so have a lot of surface area. It's far easier to get 2 x 8 lb pork butts on the grate than one 16 lb packer. Unless you plan to do large quantities at a time, the 22.5 is probably more than you need and is reall overkill for a couple of pork butts for example.
As far as using it as a grill, I think you could, but you may need to get an extra grate as there are no brakets that low on the smoker.
I have the 18.5" WSM, and while I have yet to do a brisket (ever), I'm pretty certain I could fit a packer in it. If you got the 22.5" model, not only would it make me incredibly envious, but you would probably have no problems fitting just about anything you wanted in there. The only issue with the 18.5" is that you do get some extra heat running up the sides. Putting a couple inches of foil under the ends of your food does take care of that little issue, but it can be a bit of an annoyance.
On my 18" WSM, I have done upwards of six 7-8 pound pork butts at once.