A.R.

Interesting results! I've always found that in real life the smallest and biggest focus point sizes give the least reliable results because the small one fails so often and the large one misfocuses so often. 
 
You're analyzing some very precise focus here with a very convenient subject (i.e. tons of tiny vertical lines) so it makes sense the smallest point works the best and is a good test for your purposes. 
 
Personally I'd support Fuji making a smaller focus point. It could just be an even smaller box that's smaller than the cross. That said it may not be a meaningful change, I suspect the current smallest size reflects the actual limits of precision for the AF engine, in which case a smaller box would do nothing. 
 
Hope they're working on it either way! I need to remember to be more careful about AF box size and make it smaller when there's enough light+contrast for it to still work.

Again thank you for sharing your experience! I´ve just finished some focus testing for 18mm and 23mm focals with a tripod. Testing both, the smaller square focus and the step up bigger square focus.
 
At 18mm the focus is clearly better with the smaller square. The same at 23mm. And this is with a tripod and still object... shooting with handheld and people... this gets much worse. So I still think we need a smaller square.
 
Here is the test, there is a quality lose in both ways, but if you can´t see it at 100% size try this: http://1drv.ms/1MsPAmM
 

 
Edit: I don´t own the 10-24 anymore,  but I tested a few weeks ago and as long as I was going wider with the focal lenght the focus lose got worse, that´s why I gave it back. (Fuji please... smaller points   )
 
Regards!

probably.
 
My experience is also that using the smallest of the size is what makes lenses like the 60mm be less precise rather than more! The smallest of sizes causes the lens not to be able to focus. Put it one step up and all works a lot better.
 
I had the  10-24 which is the shortest of the AF lenses and I’ve never felt the need for a smaller af point!  Now I have the 12mm Samyang ( manual focus) which I rarely focus!