It's really more dependent on genetics and environment. A well fed, healthy snake will have more energy to put towards growth, but will still be limited by it's genetics. As an example, eastern garters (T.s.sirtalis) can get well over 3ft., especially ones coming out of Fla. Where as I know Felzer had a group of individuals from VA that were topping out at less then 2ft. Also sex comes into question, on average male garters tend to be a smaller then females. Your T.s.pickeringi should top out at around 2ft. for a male and 3ft. for a female, but again...that's not definite. Hope this helps.

Roy