The radial electron momentum densities I(p) of atoms are known to reveal several local maxima and minima. For the 103 atoms from H to Lr in their ground states, we report that the reciprocal momenta 1/p(max) and 1/p(min), where p(max) and p(min) are the locations of the maxima and minima in I(p), respectively, have good linear correlations with the relative sizes R of atoms, defined based on the spherically averaged densities rho(r) in position space.