Enough to throw a couple of days of clothes into, plus miscellaneous other odds and ends (including lights). I'm planning to be flying a bit and would prefer to avoid check-in if possible. So this pack has to be airline legal.
I don't need something that fancy, just spacious. No special features inside. In fact, forget all the crap 'laptop' compartments. I treat neither my laptops nor cameras with kid gloves. For that matter, forget camera compartments too, they just tie up valuable space. A big hollow internal inside would be great
I'm not looking at one of those gigantic backpacker bags. A 'hollow' version of the Fastpack 350 or similar size would be something along the lines of what I'm thinking about:
Comfortable on the shoulders would be important too. Weight of the pack unloaded should be as light as possible. I've seen camera bags that weigh six pounds dry - with airlines actively WEIGHING carry-on these days, that eats up a lot of your seven permitted kilos.