Jonathan,
I suggest 107cm is about right, which is what the carbon ones are supplied as.
Any longer starts to catch the mainsheet falls too easily, thereby effecting your steering overly in tacks.
Any shorter will start to effect your two handed playing of the mainsheet when hiking from the front of the cockpit.
I don't see this as being dependent on size of sailor.
There is another bimble the aluminium extensions benefit from;
The shafts can be slippery with no matted 'non slip' or 'ribbing' on them.
Simply wrap some tape around them in a couple of places so your hand can not slip on the shaft.
Without this I collided into the original Aero #1001 last winter and Karlos dropped his rounding a mark in Tallinn last weekend.