Suppose only the two 1s electrons shield the outer electrons of period 2, so S = 2…

Atomic radius falls across a period, because the nuclear charge grows while new electrons enter the same shell. It grows down a group, because each period adds a shell. A cation is smaller than its atom, an anion larger. In isoelectronic species, more protons mean a smaller particle. The covalent radius is half the distance between two bonded nuclei, and the metallic radius half the distance between neighbouring nuclei in a metal. In a simple model, the radius is inversely proportional to the effective nuclear charge, $Z_{\text{eff}} = Z - S$.