How do you arrange the following atoms in order of increasing atomic radius: Na, Be ,Mg?
Atomic radius decreases across a Period.........
Atomic radius decreases across a Period from left to right as we face the Table, and INCREASES down a Group. You can use the Periodic Table for immediate ordering of atomic radii on this simple basis/
Second Period beryllium is definitely smaller than third Period magnesium, but Group 2 metal magnesium is definitely smaller than Group I metal sodium.
So in order of increasing atomic radius:
See this site for actual numbers. As a chemist, as a physical scientist, you should always have a handle on the relevant data.