Easy to read up on the Avalung option on the BD site + BC oriented sites. May or may not be a good choice depending on where you are, the terrain you will be in, and what you are doing...
Again, if you are thinking quick side-country laps, "small" is likely OK enough to cover the basics.
However... while I only skin now and again - here is the pattern for most days:
Start out feeling a bit chilly...
+5 minutes - open zippers
+10 minutes - shell comes off & into pack
+15 minutes - fleece comes off & into pack
Skin along in base layer or t-shirt (even down in low twenties or teens)
Stop to rest and snack (suck down ton of water all along)
+1 minute - layers back on
+ 5 minutes - start again
Kind of repeat process...
If you are going to do the snowshoe thing, I'd expect something roughly similar. So think in terms of a pack that'll hold just about all your top layers down to your skin - as well as lunch, water, helmet and goggles, small emergency kit, spare gloves, and cap, etc. Plus the stuff you initially mentioned. Of course YMMV
Google for reviews on sites focused on these kinds of packs. And look at reviews on retail sites.
Knowing nothing of your background, the usual admonitions about training apply...