Betta breeders need your help?

i know juvenile male bettas need to be seperated at 3 months of age. what is the minimum size cup you keep baby male bettas in? i have heard i can use teh cups that adult bettas are sold in at petstores, but they are so tiny. is that really ok? i dont' see how anything could grow in literally a cup of water. i'm trying to collect only containers that are bigger than that. also,are milk jugs with the tops cut off for more surface area ok to use?