I read this thread, then realized it was an oldie. Two thoughts; first of course is the ol' fish story. 5' garters eh? The narcisse dens in Manitoba are one of the most studied snake populations on the planet. If you're regularly finding something that isn't matching the documented science, then reconsider your methods. Generally, gartersnakes can be stretched along a measuring tape or ruler.
Second, there is growing evidence that human pressures favor smaller snake size. Larger snakes are more likely to be hit by a car, killed by snake haters, or collected for a pet. The population that I studied this spring had an average size 10-15cm shorter than 15 years ago. Max length was also very much reduced.