Atomic radius increases an moving from top to bottom in a group because as the value of principal quantum number (n) increases, new shells are added. Therefore, the valence electrons move away from the nucleus because internal energy levels remain filled with electrons, due to which shielding effect the nuclear force of attraction reduces.
Atomic radius decreases while moving from left to right in period because the shell remains the same and electrons keep filling in the same shell but the nuclear charge increases due to which the force of nuclear attraction increases.