ok if were talking minimum, then a 300x200x200 will do for a trio, not the best of conditions but we are talking minimum! for fry the same thing will do each of these tanks cost about $25ea.
i would never do this becuase of three main resons, the first beeing the fact that sunted growth will occure
second very limitied space, and third cleaning and water changes would have to be done very regularly.
i would suggest atleast four two foot tanks:
first 2 foot tank: for the pair or trio you want to breed
second 2 foot: for the continous flow of babies that will come from the adult females, and this way they will grow faster and no stunting will occure.
third & fourth 2 foot: seperating the fry once they have matured into sexes ( e.g males in one tank, females in the other)
this way you can pick which guppys you would like to breed, depending on what desired patern, coulration and body type you wont. and then they can be put into the first two foot for breeding. this also means you can chose how the strain turns out!
Shae 250