The electron-pair intracule (relative motion) h(u) and extracule (center-of-mass motion) d(R) densities represent probability densities for the interelectronic distance and the center-of-mass radius of any pairs of electrons, respectively. For 102 atoms from He (atomic number Z=2) to Lr (Z=103), we report that electron-pair radii R-2i and R-2e, defined by h(R-2i)=c(2i) and d(R-2e)=c(2e), have good linear correlations with the relative sizes R-1 of atoms introduced based on the single-electron density rho(r) such that rho(R-1)=c(1), where c(1), c(2i), and c(2e) are constants common to the 102 atoms. It is also shown that an interesting relation R(2e)congruent to R-2i/2 holds, if c(2e) is set equal to 8c(2i). (C) 2000 American Institute of Physics. [S0021- 9606(00)30316-6].