How many gallons go in a gas tank? Nobody can answer what size your fittings are without seeing them. They are probably 1/4", 3/8", 1/2", 3/4" or 1". I suggest you go to the hardware store and look at some pipe fittings to compare the thread size with what you can see. Or just buy plugs in each of those sizes and return the ones you don't use.
metalmagpie