Personally I must have something at least 24" wide by 12" deep. Height can be anywhere from 12"-15" Most of the backpacking type of sleeping bags and pads are roughly 12" long (at least the dozen or so different items I have). It just helps make packing easier. Of the 20 or more MC tailbags, duffels, saddlebgas, tankbags, e.t.c. I own. For long trips my rear seat duffel of choice has always been the Cabelas boundary waters duffel. 24x12x12 rectangular. rectangular instead of round gives you roughly a 35% increase in volume compared to the same length and width\height of a round duffel. Plus it makes packing much much easier. Also, it's doesn't roll away on ya.