Good question, right?
The old Chinese answer is the length of a real fighting stick should be up to the user's eye-brow, whatever his height; this was in the days when it was actually used for fighting and not demonstrations. Lift the weapon up a couple of inches and it will fall just between your foot and the top of your head.
Reason? If the stick is higher than you by a foot, your techniques will be restricted largely to upper parries, straight thrusts and horizontal strikes. High speed vertical swings and twirls will mean constantly hitting the ground and thus require constant adjustments of hand positions -- something you cannot afford to do in a real fight.
There was no change of length; only students (tall or short) having to use what was available in the dojo, regardless of the students' height. But if you were to use the stick as a personal weapon as an everyday survival tool, carrying a 7 feet stick around is not pratical. A stick up to your eyebrow is just comfortably right for carrying around and fighting, especially if you need to enter a door.
I'll rather be happy than right, anytime.