You'd think it would work better if it matched the span of the stab?
Based on the size of the wing it does look a bit too small to me. In addition, we don't have the velocity that F3A ships generate, so you think it would even be bigger still ,but only Kenny knows whether its the right size or not. I would have to assume this is the size he arrived at, after much experimentation. Quite possiby, a bigger size could induce bad things.
What is important to know about canalizers is that, it is not just their size, but also their proximity to the main lifting surface-the wing.