My DH has the large Carradice bag, as well as a medium sized one. We don't tour with our carbon road bikes, but he has used the large one while we were on supported tours and wanted to stop and buy stuff; also for bringing stuff from the farmer's market. It attaches with a special clamp to the seat post. I know that this is a no-no to most people, but it has never caused a problem. We have very light Kuotas; DH's bike is 52 cm and the large bag fits well.
Me, on the other hand, can barely find a tiny bag that will fit on my tiny bike.