I purchased the X100 new at the end of 2012, had no idea that the 100s was around the corner. Anyway I enjoy the camera very much and have been using it on assignments. Here is the question although I should really should figure it out myself but maybe there is a short cut asking you. I have been using center focus area selecting the smallest area as with my dslr. I still on occasion to my surprise have out of focus but background sharp on the least challenging shots,head shots that are close using horizontal frame. I have since changed the center area to the largest point but have no way of knowing until I run it through it's paces in real world situations. So you experienced ones, what have you found to be the best? Is bigger better?
I think it is situational. Large has a better chance of finding contrast and locking focus, but is also more likely to lock on something other than your intended subject, if the box covers more than your subject. Smaller will be more accurate since you can place the point exactly where you want to focus, but is also more likely to fail or hunt in low light.
I find the default size to be fine for most situations, and altering the size as needed depending on the scene.