I have a Cortech duffel that measures roughly 12x20x10 and isn't waterproof. It is fairly easy to pack and access the contents while on the bike but I'd like something a bit bigger (12x24x12 - 14x26x14 range?) and waterproof. I searched and read several threads but still have some questions. First is with sizing. How is that bags with almost the exact same measurements can be rated anywhere from 40-60L for example? Without being about to see them in person, how do you know how much the bag will hold? Next. The bag will be used for carrying all sorts of stuff (clothes, shoes, fly fishing gear, electronics, etc.) but not camping gear which is mostly what I read about. I want something that is easy to pack and though I won't be in it at every stop, something that I can access if needed while on the bike. Anyone have recommendations?