Periodicity in Atomic Size

In homonuclear diatomic molecules the distance from the centre of one nucleus to the centre of another nucleus gives the bond length and half of this bond length is atomic radius.

Period

The first member of each period is the largest in size. This is due to the reason that the extra charge on the nucleus draws the electrons inward resulting in smaller size for the atoms under reference. This trend of decrease in size of atoms, continues from left to right. However, there may be some exceptions.

Group

In going down the group of elements (in any particular column) the atomic size increases at each step. This increase is due to a new electron shell being added, when you pass from one element to another in a group.