A few people on Ravelry said their's came out a little bigger than expected. I That's why we have to do a gauge before we start. I measured the bottom of the back of mine to a sweater that I wear now and that fits me well and so far so good. I am using smaller needles than the book suggest.
I'd say do your gauge and then work from there. I have about six inches done after the decreases made on the sides for the underarms so this is moving along quickly.
Daisieknit, You could just make it in a smaller size and that should work out.