redtail cats are rapid growers. a 2'' speciman can attain 18+ inches in a year if fed correctly and lots of waterchanges. and in the wild can grow to 1.5metres. probably 36'' max in captivity.
They can eat stuff easily that is half their body size or more. You swear there is no way, but they usually try it and succeed. Figure they'll grow 2" a month no problem. For example, my big guys are 1.5 years old and 24" or larger.