Houseflies are about the size of, well, regular houseflies that you find buzzing around in the summer.
Bottle flies are a little bigger. They're the ones that have a an iridescent abdomen, usually green, but I think that have other colors too. Their lifespan isn't too long, but I've never had any survive my chams, so it would be hard to say.
AFH is right about the size, bottle flies are about three times the size of a housefly. The houseflies live for about a week and the bottle flies live about a month. I hatch them on a regular basis and have for about six months. I find buying the larvae a much easier way of trying to stagger the hatch rates. good luck . David