I agree with Craig. After a while, they fit. However, I use foil to protect the small end of the brisket from the heat around the edges until it fits. I remember seeing that tip somewhere here on the forum.
Although at first glance a packer doesn't seem to fit, I have found that you can push in the ends with both hands and compress it enough to get it to fit.
13# is my ideal brisket weight. Trim, and tuck some of the large pieces of fat under the ends. Works like a charm. Also, you should try the wet rub mentioned on this site.
I haven't cooked anything under 11# yet. I would much rather cook a bigger brisket as I have found that they just seem to taste better when cooked and cut properly.
I have been able to go up to 14 lb briskets. I have two pieces (one for each rack) of a 2x4 that I cut about 10" long and wrapped in foil. I bow the brisket over the 2x4 until it shrinks enough to fit. Works great for me.