“An experimental laboratory study of the deposition of droplets on dead house-flies (Musca domestica L.) was made, using a spinning-top sprayer to produce a spray of uniformly sized oil droplets and a cascade impactor to measure the concentration of the spray of droplets, which were dyed. The deposits obtained on a dead house-fly and a cascade-impactor slide when these were exposed in turn to a wind of 1 m. per sec. in a wind tunnel were compared colorimetrically, and determinations thus made of the collection efficiency of the flies, defined as the volume of liquid deposited on an object expressed as a percentage of the volume that would have passed through the same cross-section as the object had that not been there.”
The answer, it turned out, is around 20–40μ. See: The Deposition of Airborne Droplets on dead House-flies (Musca domestica L.) in the Bulletin of Entomological Research, Volume 50, Issue 2, August 1959 , pp. 327-332
Creative Commons photo courtesy Ismael Villafranco, on flickr