We are dubious about the scientific merits of this scheme since it requires unproven assumptions to be made about the data. It does, however, produce very pretty images. The tip should be a millimeter or so smaller than the sample so that the sample appears to the tip as an infinite plane. If the sample is smaller than the tip, fringe fields can cause spurious readings. Three tips are provided with each KP as standard.
Different tips are easy to fabricate and exchange.
Smaller tips can be made, no problem. The tip-sample interface forms a plane parallel capacitor. The distance between the tip and sample, and the area of the tip, among other values, determine the capacitance. In use, the tip-sample distance is reduced until a useable signal can be measured and that signal intensity is a function of total capacitance. That signal is composed of both the desirable values and stray capacitances from things like fringe fields around the circumference of the tip.