Smallest touch that can be detected on smartphone touchscreens
There are actually two different questions here.


Reported touch resolution - which is when the controller reports x = 123 vs x = 124 and how large is the distance delta x.  Generally this is around one screen pixel since there isn't much value visually in reporting sub-pixel changes.
Smallest finger/object capture - this is actually related to the size and strength of capacitance of the object.  Generally 5-7mm is the normal diameter of object that panels are tuned for (some are starting to look lower for stylus).  But don't confuse that size with minimum resolution.  Most (probably all but algorithms are generally proprietary) solutions actually generate some form of centroid and report the center as the touch position so a giant thumb and a tiny pinky can have the same reported resolution because both report a position based on a calculated center of mass.  


The difference is the capacitive response of the different sizes may create issues with successful detection under all conditions.