What does this number measure?
The simplest atom: one proton and one electron. This reference corresponds to two Bohr radii. Orbital depictions are conventions; in quantum mechanics, the electron is described by a probability distribution.
Compare the stated dimensions
The comparison uses the number in each fact sheet: altitude, height, diameter, length or distance, depending on the reference. Two values in metres do not necessarily describe the same property.
Source and limitations
Catalogue order of magnitude; a source is provided, but this measurement has not been individually audited.
Hydrogen: an evocation of the spherical electron probability cloud and a central proton. Densities, colours and proportions are interpreted; the dimension reference is two Bohr radii, without a rigid atomic boundary.