You'll need a pack that will carry your regular hiking gear, plus your cameras/lenses/tripod, etc. The total weight will depend on the time of year, location, and what camera gear you decide to take. Can you provide more info?

For the most part, I usually recommend a 4000 cubic inch pack (~60 liters) for year round hiking on the southern A.T. That provides enough room for winter clothing, or a large food bag, etc. It's a little on the large side, but there are plenty of good, reasonably priced 4000ci packs in the <3 pound range these days, so it's a good start. My regular pack can easily handle a DSLR and a couple of lenses, and a small carbon tripod.

For a lot of camera gear, a big tripod, heavy lenses, etc., I would want to take a much more robust pack, something like my old Gregory Shasta. More for the load carrying ability than the size, though.