What is the reasonable size for a coffee ground particle? I am looking at the following coffee grinders: Macap MXK conical burr grinder, Mazzer Robur conical burr grinder, and Mazzer Super Jolly flat burr grinder that supposedly produce a low particle size of the ground coffee. I came across an experiment (here) that showed the ground coffee particle size after being placed in water was about 10 micron (in diameter) on average. This seemed dubious to me and the post hasn't been active in 9 years so I was wondering if anyone knows if this is reasonable and if you have any recommendation for coffee grinders that produce a low particle size?
You're mis-interpreting the significance of the results in the cited experiment. There were also statistically controlled follow up experiments (e.g. here) that studied grinder particle size distribution in more detail on the same site. 

Consequently even when very precise uni-modal particle size regulation is achieved as is the case with industrial roller miller grinders, particle sizes must be deliberately blended for espresso grinds to achieve the required bi-modal distribution.

This bi-modal size distribution is crucial for proper espresso extraction where the coffee bed (espresso puck) is subject to approx 10 bar water pressure. 

Your intent in asking for grinder recommendations capable of producing small particle sizes appears to be based on the mistaken assumption that this is the normal working range for grinders. It is not. A good espresso grinder needs to have a well-defined bi-modal distribution with the largest volume of coarse (approx 500u)  and a smaller volume of fine (approx 50u) particles. And a bulk (brewed coffee) grinder should have a tighter unimodal distribution and a working range of about 100 - 1000u. 

The first plot above is the particle size distribution by volume in the OP cited experiment showing the characteristic espresso grinder bi-modal curve with average particle size peaks at approximately 500u and 50u. The second graph shows the bi-modal distribution for a Robur espresso grinder from a subsequent experiment (TGPII, Robur). And the third graph shows a high resolution plot from a instrumentation manufacturer (Sympatec).

As to the specific grinders that the OP was considering, all three are well known and respected commercial espresso grinders. FWIW, large conical grinders are easier to use and produce more consistent results. But all would work well for espresso.


Why these things matter

High quality bulk coffee grinders by contrast show a tighter, uni-modal size distribution at the selected size. The usual working range for brewed coffee is about 100u - 900u (Turkish - French press).


  the ground coffee particle size after being placed in water was about
  10 micron (in diameter) on average


10u can only be said to be the average particle size by count. It must be noted however that the proportion of 10u particles by volume is minuscule. A meaningful characterization of average particle size needs to be done by volume or distribution, the former has a strong correlation with expert taste tests.