Electrons in shells with higher principal quantum numbers are on average farther from the nucleus.
The cartoon shows a slice through an atom that has electrons in shells 1, 2 and 3.
The size of an atom depends on how many shells are occupied and on the effective nuclear charge at the outer electrons. These two effects oppose one another.
When comparing two atoms, the atom with more shells occupied is larger.
This means that atom size increases going down a group in the periodic table.
Group 1: Li (1s2 2s1) is smaller than Na (1s2 2s2p6 3s1) which is smaller than K (1s2 2s2p6 3s2p6 4s1).
However the increase going down the group is smaller than would be anticipated due to the increase in the nuclear charge.
The increase in effective nuclear charge down a group is smaller than the actual increase in nuclear charge because of shielding.
The outer electrons are shielded from the nuclear charge both by electrons in inner shells and electrons in the same shell.
Electrons in shells closer to the nucleus contribute more to the reduction of nuclear charge (shielding) at an outer electron than electrons in the same shell.
This is common sense because an electron in an inner shell is more likely to be between the nucleus and and electrons in an outer shell.
A set of guidelines known as Slater's rules puts this on a quantitative basis, but the numbers are not as important as the idea.
Each electron in a shell closer to the nucleus cancels roughly 0.85 of the charge on one proton.
Each electron in the same shell cancels roughly 0.35 of the charge on one proton.
Each electron in the same shell cancels roughly 0.35 of the charge on one proton.
An estimate of the effective nuclear charge on the outer electron on Li (1s2 2s1) is +3 - 2 × 0.85 = +1.6 (instead of a nuclear charge of +3 due to 3 protons)
An estmate of the effective nuclear charge on the outer electron on Na (1s2 2s2 2p6 3s1) is +11 - 10 × 0.85 = +2.5 (instead of a nuclear charge +11 due to 11 protons).
The contraction due to increased effective nuclear charge from Li to Na is small in comparison to the expansion due to Na having more electron shells.
An estmate of the effective nuclear charge on the outer electron on Na (1s2 2s2 2p6 3s1) is +11 - 10 × 0.85 = +2.5 (instead of a nuclear charge +11 due to 11 protons).
The contraction due to increased effective nuclear charge from Li to Na is small in comparison to the expansion due to Na having more electron shells.