Your pack must be large enough for a winter sleeping bag and real insulation under you, camp clothes and a tent. I have a 6500 cubic inch pack and my winter sleeping bag fills half or more of it when stuffed in, making very little room for down bibs and down winter coat. You almost cannot have too large of winter pack, but as TomD says - it still has to fit you.
Jim grin
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These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.