I know it probably would be a bit big (given the size chart) but at 5'5--long arms but a short torso-- and 120 lbs , would a small be baggy given the famous Arc'terx fit? I have the Marmot Exum for reference, and it fits well lengthwise but has a bit of excess around my chest and arms which isn't bad but I am always looking for a better fit and I am tempted by the burlier fabric on the Beta AR. Anybody?

6Ft ~200 43R suit, 34/35 16-16.5 shirt, 33 waist. I wear a medium Theta AR and large in the Alpha/Beta

Thanks for the input! I have a 36 chest which falls a bit below the specs so I think I will pass unless a screaming deal comes up on a woman's size.

First, I haven't personally tried on the Beta AR, but I have owned the Theta AR and the Alpha LT, both of which are also from Arcteryx's ascent collection. I wear Arcteryx size Medium in these jackets even though my chest size is about an inch greater than what is listed for a medium in the sizing chart. You don't say what your chest measurement is, but if you are significantly below the measurement for the size small, this is going to be baggy on you - the jacket is still going to have a cut that allows people at (or just above) the upper size limit to fit some layers underneath. My wife is the same height as you (and weighs only a little less) and a woman's size small fits her perfectly. I know it wouldn't be the most popular option, but you might at least want to try a woman's jacket to see if it might fit you correctly. Arcteryx jackets are awesome because of their quality and their fit, and I'd personally hate to spend this sort of money and not have the jacket fit right. By the way, while I have no experience with the Marmot Exum, the Arcteryx Pro Shell fabric is incredible - I can't imagine you being disappointed with it.