The 511 bag is a really good size for a small "Possibles type" day bag. You can carry a 1liter bottle with cup in the side pouch but it does bulge into the inside area of the bag. It will as I said also carry a small cook kit and a single meal (or two if it's compact enough).

For instance, IF I had a "Pot" large enough I could carry the water, cookset and a Knorr Rice or pasta Side dish (packaged meal like Mt. house only different)that serves two.