nice set up you got there.
RBP can grow up to 7-8" with in the first year of their age and slowly grow 1" every 3 month or so. Ofcourse its all depending on how big is your tank, how well you maintain the tank and how will you keep the well feed.
If you has plenty of room for them to swim, feed them everyday and they always eat and not a picky eater, active all the time they can grow so fast.