Jeremy Cote wrote an answer about Arc'teryx Alpha SV Jacket - Men's on March 11, 2013
My preference is to stay with the same sizing for all my Arc'Teryx jackets and mid layers, since they are cut and designed to work together.
That said, you could try a L and it would probably work, depending on your body specs, but in my opinion you are limiting the use of the jacket, especially in extreme cold situations, since you wouldn't be able to layer as much underneath it. Now that is fine if you carry a warm/waterproof piece that you are going to slip on over your shell at rest/camp, etc., but really, when I wear just an Atom LT under the jacket, I simply cinch the Alpha SV down around the chest and at the waist and that keeps the dead air space to a minimum.
I will add the only time I go with just the Atom LT under this shell is when I'm really working hard in extreme weather, and would over heat with another mid-layer on (Atom SV, for example), but this type of usage is the exception and not the norm, since if I'm fully insulated (Atom LT + Atom SV) and I need to dump heat I just open my shell and the pit zips.
SO you have to ask yourself, do you really want an Alpha SV bomb proof jacket, one that is desinged to protect you from extreme weather, in a size that limits the number of layers you can comfortably wear underneath it?