OR Helium 2

Looking for some advice on the helium 2. I need to size it so it can fit over a size Large Arc'teryx Atom LT puffy jacket as an outer lightweight shell. Anyone have experience layering with this piece?

nobody local is carrying the Helium 2 at this time so i am going to have to order it online.

Thanks for any help
Jeremy

It's a pretty slim/short fit, I am usually a Md with Lg length arms and was happy to get a Lg. If I wanted a puffy underneath I would need an XL.