My go-to packs are generally the 50-liter model, so around 3,000 - 3,500 cubic inches. (Some "50" packs, like the Atmos, are slightly more capacity in the longer torso lengths, thus the range of capacity. Others only count the main packbag, not the side pockets or lid, which also skews comparability.)

That's plenty for three seasons - sometimes almost too much in hot weather - but in winter, if I'm going to be out for more than 1 or 2 nights, I either add extra strap-on pockets or end up having to start out with my sleeping pad and its chair kit lashed to the outside, depending on the pack. (Yep, I said chair kit. I no longer consider it a luxury to have a comfortable seat; my back relaxes better, so I sleep better.)