What is the smallest element in the second period?
In the same period diameter of atom depends on how many electrons are in outer layer. So if you look at the periodic table from left to right, amount of electrons are rising in outer layer and the thrust between nucleus and electrons is rising too. So that's why atom on the right is smaller than atom on the left.
But it's not neon because the noble gases are actually the largest of their respective periods.
So I think you should note that atomic radius decreases from left to right and increases from top to bottom.