mike franks wrote an answer about Gregory Baltoro 75 Backpack - 4455-4760cu in on January 28, 2013
I have a 2L bladder in my pack...It has enough room that I could fit a 3L in the bladder bag. The bladder bag compartment is not tied down at the base and can move in the pack itself. Only the top of the bladder compartment it tied to the frame. If you move to a 3L bladder you will lose some a pretty good amount of space in your bag.