Does atomic size decrease down the group?

Hint: We need to know that atomic size is the distance between the centre of the nucleus of an atom and its outermost shell. The atomic radius is defined as the shortest distance between the atom's nuclei and the outermost shell of the atom. S block elements have metals in them while p block elements have all metals, non-metals and metalloids.

Complete answer:
We need to know that in the periodic table, the elements are arranged in the order of increasing atomic number. Atomic size depends upon the effective nuclear charge which is further dependent on the atomic number that helps us in identifying the atomic size. When we look in the periodic table, the trend for atomic size is usually like this. Atomic radius increases down the group and decreases across a period from moving left to right. Atomic number of sodium is $11$ whereas the atomic number of potassium is $19$. They both are present in the same group, i.e. Alkali metals. On moving down the group, the number of shells or orbitals increases like for sodium it has \[3{s^1}\] orbital but for potassium it has \[4{s^1}\] orbital, which increases the net effective nuclear charge and thus increases the atomic size. For more understanding we can have a look at electronic configuration for both the atoms:
\[Na\]: \[1{s^2}2{s^2}2{p^6}3{s^1}\]
 \[K\]: \[1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^1}\]
Thus we can conclude that on increasing down the group atomic size increases while on moving across the period atomic size decreases.

Note:
We have to know that the atomic radius of atoms generally decreases from left to right across a period. There are some small exceptions, such as the oxygen radius being slightly greater than the nitrogen radius. Within a period, protons are added to the nucleus as electrons are being added to the same principal energy level.