The atomic radius increases as we go down a group/family on a periodic table.
Step-by-step explanation:
"Atomic radius" is defined as the "one-half of the distance" between the "nuclei of two atoms". While moving down the group in a periodic table there is an "increase in quantum number n" which allows the valence electrons to occupy on higher levels. This shows that the valence electrons move away from the nucleus, which makes them to be loosely held. Thus, atomic radius is large.
Similarly while moving across the periodic table from "left to right" the atomic radius gradually decreases.