6-7" is around max size, with the red coming in more as they get larger. They grow slowly, just like all the other serras, about .75 to 1 inch a year on average. Maybe even a little less, considering the smaller max size.
Great piranha for someone who doesn't have room for a larger tank, plus they can be pretty nasty attitude wise.
Personal experience: I bought a red sanchezi from aquascape about 2 years ago at 3 inches. In 2 years, it has grown an inch (serra = slow growth rate) on a varied diet of raw shrimp, pellets, and tilapia.
Research implies that they will grow a max of 6-7 inches in captivity.