Depends on how small your stuff packs down. I can fit everything for an overnight in what you see in this picture, no backpack needed - sleeping bag and pad on the handlebars, water and some food in the framebag, food and layers in the seatbag. I didn't bring a tent on that ride but it could fit in my handlebar roll if I used my summer sleeping bag, or on top of it otherwise. With a gas tank and jerry can I could probably go another day or two, with feedbags probably 4-5 days.
For longer trips I am going to bring a fanny pack instead of a backpack. I haven't made it yet, but it will be able to be packed down really small into one of my bike bags, so that I can pack it away after the first day or two of a ride.
For the Idaho hot springs route this summer i plan to just bring panniers, because I don't want to try to carry beer in bikepacking bags *or* in a backpack.
If I do bring a backpack bikepacking it'll be just a small camelbak, I don't even know the size of mine. Maybe 10-15 liters? Just for layers, and i would try to put most of my heavy stuff on the bike. This is just me though - I'd really rather avoid a backpack at all, and I put quite a lot of effort into packing small (silnylon tent, Ti mug instead of a pot, tiny dropper bottles for aquamira, etc).
