hamfist
Fish Herder
If they are just being kept as pets then my personal opinion is nothing less that 5gal. It makes a nice compromise between space for the fish and maintenance levels/amount of space the tank takes up.
And just to answer the original post, giant bettas are just line bred betta splendens. You can breed for all sorts of characteristics, normally you see them bred for colour, giants have been bred for size
Treat them exactly the same as you would any other betta.
Yes, yes, yes and yes. All good info !