does really matter on size as long as they are sexually matured. I think mine did at around that size. if you keep the condition great and as long as they pair up they'll breed. some say better to simulate wet/dry season, that would trigger it but piranhas can also breed in large man made lake caused by dam, which they won't experience wet/dry season as in the wild river system, at least not as dramatic.
you can stop change water for a while, up the heat and lower the water level. this simulates dry season. (low water line, high heat caused by the sun and bad water condition) also try to lower oxygen level if you want to go extreme. just make sure they don't die. then slowly add more water back into the tank, lower the temperature and make sure all water parameters are good. this simulates wet season.
I had 7 rbps and they bred in a 45g, I wasn't even trying. maybe I just got lucky.
as long as you have male and female, eventually they'll pair up..I think.