baby doll size patterns usually fit- not the 18 american girl doll patterns (the stuffed animals are fatter), but the ones for 16-18 inch baby dolls - the build-a-bears can vary greatly in size- we have a few around..... we have a kitty (not the hello kitty, but the one with the blue eyes) and her measurements are:
armpit-to-armpit 8 inches (close to 16 inches all the way around)
chin to belly button about 4 1/2 inches
top arm about 4 inches
underarm from pit to wrist about 3 inches
yoke/ across the shoulder is about 7 inches
our stuffed dog (white with brown patches) is slightly smaller, about 1/2 to 1 inch smaller on all measurements.
if you have a michael's near you, they sell knock offs of the bear clothes that you could use to judge sizes and measurements...
we have gotten 2 sweaters as gifts that fit the b-a-b animals and they were originally baby doll patterns. I've seen lots of cute ones at knittingpatterncentral (free) and
e-patternscentral (not free)