Hi, I am a semi professional Photographer who's been hiking with my camera gear for about a year now. I am looking to up the anti. I want to start by doing about a hundred miles of the Appalachian trail. I am wondering how big a bag I am going to need. I know I want a frame or internal frame bag and it has to be big enough to carry my bivi sack sleeping bag and atleast 2 lenses and an SLR and tripod. How big should I go? 4000Ci, 5400 Ci, 6000Ci?