Originally Posted by tahonng
So you got the 40?
Yes, I have a 40 in the 3a (lumber cardigan) and it fits fine. I would probably be fine going up to a 42 as well for a slouchier fit. So, I would guess that as a J. Crew small you would be fine in either a 38 or 40. If you are on the large side of a J. Crew small, then you might need to stretch the 38 a slight bit for it to fit well, but I doubt that would be a problem (based off of my own experience at least).
Note, that my experience only applies to their cardigans. I believe the crewneck sweaters are supposed to fit differently (I think a bit looser than the same size in a cardigan but I may be wrong).