Which element has the smallest atomic radius in the fourth period?
Atomic size decreases across a Period from left to right as we face the Table, and INCREASES down a Group, a vertical column of the Table.
This is a consequence of two factors: (i) nuclear charge; and (ii) shielding by other electrons. Incomplete electronic shells shield nuclear charge very ineffectively, and thus atomic radii decrease markedly across the Period to give the Noble Gases as the smallest atoms of their Period. Down the Group, the complete valence shell shields the nuclear charge, and the electron is added to a new shell, and radii increase.
As a chemist, as a physical scientist, you should seek some numbers to inform your argument. Here is a start. Do these data support what we havwe argued?