They should definitely be big enough to breed, I have a few in my pond that size and I got several babies this year. But I wouldn't have a clue on how to help you breed them indoors. How big is your tank?
i always breed comets in aquaria. seperate the male and female for two weeks. feed them well and do water changes regularly.
then put them in an aquarium with some spawning mops. they should breed in one day or two.