i plan to use it on my roofrack, most likely extended 4wd trips when the rest of the car is full.. It would just let me put bulky, not so heavy stuff up out of the way. Sleeping bags, pillows, clothes, but you could put just about anything in there really, i try to keep the weight down low so bulky but light stuff for me.
I went to a camping shop after to see what they had and i found a similar shaped bag, but a bit smaller, (didnt have the straps etc) and i think it was 65 litres. Visually, you guess you could probably get 3 largish sleeping bags in it, and probably then some...........i could be wrong, im bad at measurements.....rest assured its a good size!