How would you rank the following elements in decreasing order according to atomic radius: Cs, Na, Li, Rb, K?
Atomic radius decreases across a Period, but increases down a Group. So find a Periodic Table!
All of these metals come from 1 particular Group of the Periodic Table. A Periodic Table will be supplied in every examination in Chemistry and Physics you will ever sit. It is your opportunity now to learn how to use it.