Well, triporcatus can grow to be big...in the wild. But I haven't seen many captive raised ones that were much over 10 inches. Maybe in time and a lot of room they might reach 12 inches. I raised one for about four years that was a female and she grew to be about 8 inches and then really slowed down. A wonderful animal though!
S. salvinii is smaller.
Both make fantastic display turtles and are very responsive.