Several sources cite specific quantities in the order of 0.1 – 0.5 nanometers for the usual set of atoms. Are these estimations or completely accurate numbers? Does every atom of the same element share an exact size?
Measuring atoms could lead to think in size, but why not also weight? The elementary particles need to be determined as well thus some mechanism takes place for each value.
Different scientists seem to use the trick of focusing large quantities and known weights, size and density in liquids to estimate the individual values by division.
Other technique appears using scattering, guessing that if they are touching the distance equals the size or near. After using the scattering, with a bit of trigonometry the feat could be done.
As of today, no machine or direct tool helps in this matter, so every number has then a component of supposition since it assumes conditions that perhaps aren't unarguably true.