Actually its just over 2.5 litre, so must be 3/4 gallons.
Not quite. Standard tanks are somewhat under 1/2 gal when level is at bottom of the fill tube.
They do sell 3/4 gal (3 liter) tanks and they are commonly available and cheap.
I'd like to know where to buy 1 gal teardrop tanks for a reasonable price. The one I have was $20 a few years ago but don't seem to be available anymore. Several people are begging me for one.