How smooth is peanut butter?
Madehow.com discusses the entire process of making peanut butter. 


  Peanut butter is usually made by two grinding operations. The first reduces the nuts to a medium grind and the second to a fine, smooth texture. For fine grinding, clearance between plates is about .032 inch (.08 centimeter). The second milling uses a very high-speed comminutor that has a combination cutting-shearing and attrition action and operates at 9600 rpm. This milling produces a very fine particle with a maximum size of less than 0.01 inch (.025 centimeter).
  (Emphasis mine)


However, creating the correct emulsion of fat and particles may have a greater impact. As with ice cream, the fat coats the tongue and helps prevent a grainy texture. 

EDIT:
This report from Micromeritics, a rheological analysis firm, shows the particle sizes of three (unnamed) chocolate bars. However, it has a useful graph of particle diameters. For the smoothest chocolate bar, approximately 50% of particles are smaller than 5 nanometers, and nearly 80% are less than 10. The lowest quality bar has only 25% of particles smaller than 5 nanometers, and only 45% are less than 10.