Which are the LARGEST atoms?
A fundamental trend observed for the Periodic Table, is that atomic size INCREASES DOWN a Group, a column of the Periodic Table, yet DECREASES ACROSS a Period, a row of the Periodic Table, from left to right as we face the table.
This fundamental trend is a consequence of Periodicity, and two atomic properties. In a given atom, incomplete electronic shells shield the nuclear charge VERY INEFFECTIVELY. The result, as we go across a Period, the element to the right is a SMALLER atom due to the increased (and poorly shielded) nuclear charge.
Once a valence shell is filled, shielding of the atomic charge is effective, and the next valence shell is commenced (with the alkali metals) at a larger radius, a large separation from the nuclear core.
Of course, the Table is constructed so as to reflect the fundamental electronic structure.
As a chemist, as a physical scientist, you should look at actual data to inform your argument. Here is start.