I think a gallon is ok but you really don't have a lot of room to put plants or decorations. I have a gallon and a half critter keeper and its plenty big for one of my females. She just has one little plant in there. Also, small containers need frequent water changes.
I think that females don't need quite as much room as males just because they are so much smaller, so I would say Definently no less than a gallon for a female, and 2 gallons for a male, altho more is better
I got my 2 gallon tank at petco for $24.99. It came with a undergravel filter, cover with feeding port, and light. I need to get a heater yet as it's around 74 at the highest right now. I got my tank for my birthday from my son.