Which are the smallest and largest atoms?
Well the largest is probably francium, \(Z=87\), or caesium, \(Z=55\);and the smallest is definitely helium, \(Z=2\).
As we face the Periodic Table, you must SIMPLY know that atomic size DECREASES across a Period from left to right as we face the Table, a horizontal row of the Table, and atomic size INCREASES down a Group, a vertical column of the Periodic Table.
Graphs of atomic radii make this very clear.
Two factors influence atomic size: (i) nuclear charge; and (ii) shielding by other electrons. Incomplete electronic shells (the which define the structure of the Periodic Table) shield the valence electrons VERY IMPERFECTLY from nuclear charge, with the result that across the Period the incrementally increasing atomic number and nuclear charge WINS over the electronic charge.
Once a shell is complete, atomic charge is shielded somewhat, and the next valence shell starts out at a more distant radius, and so the aufbau cycle of contraction across the Period begins again.