I also use a Cabelas boundary waters duffel, the smallest size (but called large) which is 24x12x12. I specifically wanted something at least 24" in length to accommodate a Kermit (23"), my cot (24") and my GCI macro table (24"). The 3 lay across the bottom of the bag side by side and only take up about 2-3" of height from the bag.
Added to that are an Exped Symat 9DLX, HUGE Walrus tent, a Mountain Hardwear FLIP sleeping bag, cook kit, stove inside of cookware and a single change of clothes in case I have to change in the tent. (rest of clothing is in a side bag, I also pack a thin high tech material t shirt in with my sleeping bag). If two up then there is just enough room for a second sleeping bag and pad, or I switch to a wide pad and pack is separately depending on the destination and length of stay, and since I have human warmth I switch to two ultra light summerweight sleeping bags.
So, just think about what the largest or longest item you might carry in the duffel is and go with something that will accommodate.
Oh and the duffel is fully waterPROOF, you can fill it with bricks and feathers and dump it into a pool, pull it out and all the feathers will remain dry (a moderately weighed load will float)