Atomic radius of the elements generally decreases from left to the right in a period because on moving from left to right in a period the number of shells remains the same but more and more electrons get added to the atoms and hence the effective nuclear charge increases and thereby pulling the shells closer to the nucleus and hence atomic radius decreases on moving from left to right in periodic table.

Atomic radius of the elements increases on going down the group because on going down the group there is an increase in the number of shells. As the number of shells increases, the valence shell gets farther and farther away from the nucleus resulting in reduction of overall effective nuclear charge. Therefore, the atomic size is obviously expected to increase.