Here is the relevant excerpt:
To be precise, Millikan’s pulverized oil droplets are much smaller than oil drops. Typically a droplet is 0.1 microns in radius while a drop is about 1,000 microns (1 millimeter). Since there are about 1021 atoms in one single drop of water, one droplet contains roughly 1017 atoms. So Millikan showed that the electromagnetic force exerted by one single electron could counteract the weight (i.e. the gravitational force) of 1017 atoms.
The levitation of the oil droplet is possible if the electric field it is subjected to is equal or greater than 32,100 Volts (over a few centimeters).
it's different if you throw your keys in the air:
1/ The atmospheric e-field is only about 100 V/m
2/ Unlike the electrically charged droplet (one electron), the keys are not electrically charged.
You're right that there are 4 orders of magnitude between radii R of a droplet (100nm) and that of a drop (1mm).
Number of atoms, though, in a medium of more or less uniform density, scales up/down with volume, roughly ~R^3 and not just ~R, which then brings us to a trillion droplets in a drop, i.e. 10^12 difference (3*4=12 orders of magnitude) in number of atoms between a drop and a droplet, giving us a billion atoms (10^9) in a droplet at the end.
Cross-check: size/radius of an atom is roughly 0.1nm or 1 angstrom (10^-10 m) which is 3 orders of magnitude shorter than radius/size of a droplet (100nm = 10^-7 m). That gives 3*3=9 orders of magnitude larger volume for a droplet than that of an individual atom, thus a billion atoms in a droplet and a trillion droplets in a drop give together 9+12=21 order of magnitude (10^21) larger volume in a drop than that of an atom, i.e. 10^21 atoms in a drop as cited in ECHCC.
N.B. Would you allow me to share my 'other' notes, thoughts impressions visualizations, while going through ECHCC, in smaller chunks of several chapters, or would you maybe prefer all-in-one after finishing reading whole book?