Answer:
Larger atomic radius as you go down a column.
Smaller atomic radius as you go across a period.
Step-by-step explanation:
Going down the periodic table, the atomic radius increases because each new period adds a new energy level/electron shell. New orbitals must be added outside of the pre-existing orbitals, expanding the atomic radius. For instance, the third period introduces the third energy level (3s) versus the fifth period which introduces the fifth energy level (5s).
Going across the periodic table, the atomic radius decreases because the number of protons within the nuclei increases. When there are more positively-charged protons, they have a greater pull on the negatively-charged electrons, thus brining them closer to the nucleus and shrinking the atomic radius. For instance, the atomic radius of chlorine (17 protons) is smaller than the atomic radius of the elements which lie to its left, such as phosphorus (15 protons) and sulfur (16 protons).