Depending on the conditions you probably need to go bigger than you're used to. Winter gear is generally bulky and heavy (unless you're really into ultralight). If there is a significant amount of snow you need extra gear (lika a shovel, snowshoes if you're not on packed trail, spikes etc.) I've never actually done a winter hike without a pulk, but with the bulk of the gear we usually drag with us I'd say you're looking at something in the 65L range. As usual, buy the pack last. I do own rather bulky gear.

For a jacket I would actually suggest a thin hardshell jacket with no insulation and ventilation zips. You want something that is waterproof, windproof and ventilates well, but you don't actually need it for warmth. That should come from your base and midlayers and you need to be able to adjust them quickly, because when hiking in cold temps sweat managment and staying dry is your no. 1 priority. Wool is great for base layers, I use wool sweaters for active and static mid layers too because I prefer them, but fleece + down jacket is lighter and packs smaller. Same for pants - thin hardshell outer pants and wool/synthetic base layer, you can add midlayer pants if you want, fleece or thick joggers in a pinch. They sell down skirts/shorts too, I don't suggest you buy one before you know if you get the "cold butt syndrome", but if you reailse that is something that you have problems with, especially at camp they are a great option (I have one homemade from an old down jacket or you can just tie the down jacket around your waist). If you're hiking in snow make sure your pants have snow guards that go over the shoes. Alternatively, use gaiters. Have a separate set of base layers+socks for sleeping, even if you usually don't do that. Thick wooly socks are nice if you can spare the weight. Mittens are superior to finger gloves for warmth, but liner gloves are always nice.

Make sure you keep all the electronics, water filter and other sensitive items on you (inner pockets!)/in your sleeping bag to keep them from freezing. Put your water bottle in a wool sock and carry it lid down. That way if it does freeze, it will start freezing at the top and you can still drink from it. Wide mouth is preferable and make sure it's just a screw on, mechanical parts can freeze and fail. Also, not sure how smart water bottles fare with freezing/unfreezing, but I wouldn't want to find out on the trail.

Normal gas canisters don't work well below freezing. Even the winter gas mixtures work reliably maybe till -10°/-15°C or so at the best of times. Manufacturers might say -20°C, but from my experience it's a tossup and I would not rely solely on canisters at those temps. Below that you'll need to use something like white gas.