Because your boat isn't "standard" in any way to begin with, applying any class standards may not be relevant. Measure the distance between the forward end of the tiller and the last part of the sheet that comes down from the boom. This is the absolute minimum length for an extension; it's probably not enough for effective hiking (or sitting forward in light air), but at least it's very easy to move from one tack to another, even if your tacks aren't the smoothest
The optimum extension length is bit longer - set your tiller at a 30° angle to the centreline of the boat, and measure again to the sheet (or the last sheet block on the hull).
Chances are that you'll find a "happy medium" somewhere between the above two. There are standard lengths of 91 and 107 cm, which are the most likely choices (Weston's extensions sound very short indeed ).
If your tiller is very long, shorten it instead of getting a shorter extension.