I have a 5g running with RCS. That's the tank I started them with. When the population started growing a lot, I took some and put them into a 10g. Should be fine if you have lots of plants.
There's really no minimum tank size that it will or will not breed in. I've kept a group of 20+ in a gallon tank to hold and fast for someone once. When he flaked and I was transferring them back into the the 10g, I noticed a bunch of tiny little RCS on the sides of the glass and a couple more of my females berried.
Of course, I'm not advocating keeping them in anything less than a 5g tank. They are like rabbits.