If its a roll bag as I know it you can roll/compress to what length you require, its probably litre/diameter you need to decide on which depends on usage & capacity required.
Yeh, that's what I'm worried about, if it hangs over the side like a side of beef, not interested. Good shout on the Ortlieb, thanks, I'll check it out. Off to S.O.F. for a week and back to luggage woes