If you are doing BIAB and you want to do five gallon batches, you will probably want about a ten or eleven gallon pot. You add all the water you will use at once, so you have to allow for the recipe size itself, the grain, grain absorbtion, hop absorbtion, evaporation and any other losses. So a five gallon batch calls for around eight gallons of water and when you add the grain to that the level will rise...
I brew on a burner outside so I don't have much experience indoors (the first and only time I tried it I was new to brewing and had a boilover). It sounds like a lot of people don't get enough output from their stoves and supplement them with a heatstick.