Except that your finger is bigger than that so you can't see what you're doing anyway. On ANY touch device. STYLUS (as you said) ftw. Or a mouse pointer. Infinitely small.
You are comparing size with time, which is an invalid comparison without further constraints (e.g. a visual response constrained to emanating out from the touch of a finger). Thus, you are making the assumption that all visual responses to the input of a finger will only occur directly under the real estate occupied by a finger - an assumption that I would argue is entirely unfounded in the realm of modern phones.
Agreed. To me it's either when you are at the minimum size of a numbering system with discreet quantum. Also yo ucan do a cross hair with the middle pixels empty so I guess that would be infinitely small, and a closed crosshair would be 1.