Follow the idea
Atomic radius is not a single directly measurable hard boundary; several operational definitions are used, such as covalent, metallic and van der Waals radii. The definition must be stated before comparing values. Periodic trends are useful, but different radius definitions should not be mixed. An electron in a many-electron atom experiences the attraction of the nucleus modified by shielding and electron-electron interactions. Effective nuclear charge is a useful model of the resulting attraction. It helps explain why periodic trends cannot be understood from nuclear charge alone. Inner electrons partially screen outer electrons from the full electrostatic attraction of the nucleus. Shielding, penetration and nuclear charge work together to shape orbital energies, atomic size and ionization trends.
Primary references
IUPAC Gold Book · NIST · BIPM