I've never done a ski ascent, but if your gear "just barely fits" in the 36L pack, that one sounds about right. If you're skiing, you don't want something too big flopping around on your back. Add a couple of extra straps, and you can tie your clothing layers on the outside of the pack if they won't fit inside.
Good luck on the trip. Are you trying the Mountaineers Route or Main Trail route?
