How does atomic size change within groups?
Atomic size increases as you go down groups.
The groups of the periodic table are the columns.
the Alkali Metals column 1 group I, the Alkaline Earth Metals column 2 Group II, Halogens column 17 group VIIA or the Noble Gases column 18 group VIIIA.
As you travel down a group additional electrons are part of the atom and more electron orbitals are added to the atom.
As you add 2s and 2p orbitals in the second period or energy level and and then 3s and 3p orbitals for the 3rd period (3rd energy level) and then continue to the fourth energy level and add 4s, 3d and 4p orbitals the size of the atom expands.
Hope this was helpful.