as far as I know there is some genetic component to how big garters (or anything) grows
it is certainly possible to stunt the growth of a creature to prevent them from reaching their potential size, but I doubt that even with the most optimal diet, one could surpass that genetic parameter
while I see that babies with a rodent preference grow noticeably faster than those with worm or fish preferences, I don't know that it necessarily limits their adult size not to eat rodents
to add - I believe that in the wild, food preferences may change seasonally, as well as with life and growth stages
I've noticed that some of my garters (the wc ones) will have different prey preferences at different times of the year