Well if you buy Females and Male guppies then your going to need room for the babies and the adults. I would say at least a 10 gallon for the adults (8 max guppies) and if you wanted to keep the babies as well then the best option would be to buy an additional 5-10 gallon for the growout fry.
If you buy males and females remember to buy 2 females for every 1 male that you have otherwise the male will harass and stress out the females.
If you buy just all males then you can buy a smaller aquarium. Probably 3+ gallon. Remember the 1" rule of fish per gallon so every male guppy that you have you need a gallon of space.
If you buy just all females then your still going to get fry because female guppies can hold sperm for over several months. The guppy will have 15-30 fry each month as well. So you would follow the same rule as above if you were to buy males and females.
