They grow quickly to about 3" Then grow slowly to about 5", then grow verryyy slowlllyy the rest of their life... I dont think they ever stop growing.
The biggest I've personally seen is a 10" at my LFS.
I've kept 5 in a 55 gallon for two years. But they arent as happy as the 8 I have in a 150 gallon. They are just OK. However, they enjoy roaming and are happy with a lot of space around them.
They also like sandy substrate, lots of caves, clean water and to eat a little bit 4-6 times a day instead of just twice.