Take a small one to hold your lunch, and some odds and ends. Weatherproof is best. There are bag checks at the main top stations at both mountains. There are also restaurants, lounges & bars and restrooms at the top of most of the main runs. My back pack is about 12x12x12, I carry nothing I can't do without, I just hang it on the branch of a tree at the top of the run and go ski until lunch. I have skied Whistler since it opened and I've never worried about my pack at the top of the hill. I also have a small fanny pack with my survival gear with me. My whistle is plastic and where I can reach it without my hands.
Never ski alone on that mountain. It is too big, one caught edge and you can be off the trail and may not be found til the thaw. This is true mountain skiing where the mountain will win every time. A great mountain and a great party town at the base of the lifts. Take money, it's not cheap.
