Thanks for the reply. As you might have guessed I have my perspectives on this but I want to get my thoughts crystal clear in preparation to add my single hand instructor certification to my recent spey cert. I've learned almost as much from listening as I have from doing so these discussions are valuable.
Are you interpreting "length of stroke" as only the finish to the cast? If so we are on the same page, albeit indirectly.
Perhaps the disconnect is that I do not interpret the stroke as having two separate parts. I see it as all one continuous power stroke, beginning to end, and lengthening it does not affect the size of the loop per se. Lengthening it does add momentum, power and results in extra distance but the shape of the loop strikes me as being a function of position that the rod is stopped (unloaded) relative to the path of the line coming forward past it.
For instance, if you made a very abrupt stroke and the path was circular like an analog gauge, the loop would still be very open. If you made a very long power stroke in a straight path and stopped the rod tip just a tiny bit underneath that path as you let the line zip by, the loop would be very tight.
Another example is a pumpkin in a wheelbarrow. If you pushed it a very short way into a wall, the pumpkin would not fly out any higher with a longer push into the same wall (initial vector). Certainly the acceleration and momentum would affect it's flight speed and duration. But to change it's initial vector would likely require playing around and rolling it down different hills and having it hit different walls the pumpkin could be made to fly in a variety of directions - hill angle relative to wall angle and shape / rod stop relative to line path (per Bruce Richards explanations).
However if you are saying that only that final part is the "power stroke" and not the whole casting stroke, then I see it. A short finish usually means a stop position closer to the line path, hence a tigher loop.