Answer: Increases down a group, but decreases down a period
The atomic radius usually increases while going down a group due to the addition of a new energy level (shell).
The atomic radii decreases across the Periodic Table because as the atomic number increases, the number of protons increases across the period, but the extra electrons are only added to the same quantum shell. Therefore, the effective nuclear charge towards the outermost electrons increases, drawing the outermost electrons closer. As a result, the electron cloud contracts and the atomic radii decreases.