That really depends on the shape more than the weight. I have done a 14# brisket sitting angled a bit on the rack with the flat end curled a bit (shrinkage will straighten it out pretty quick). Basically, anything about 18" or shorter (15" depth on the smoker) will fit.
I've done a 18# brisket on my #2, no problem. As said before, it's more based on shape/length than weight. I've done 13# briskets that I've had to fold up the flat a bit to get the door to close. I've done 16# briskets that had 2-3" all around them.
If you can cram it in, and get the door closed, you're good! Doesn't matter if it's 8 or 80 outside, your brisket will shrink about 50% by the time it's done!