For the following elements (Group 1A), Li, Na, K, Rb), which atom is larger overall?

The smallest atom you have listed would be lithium (Li).  

Explanation:

Lithium is the third smallest atom listed on the periodic table of the elements.  It has an atomic number of three, meaning it has three protons in it's nucleus.

Atom size (Atomic radius) increases from UP to DOWN in the same COLUMN(group) of the periodic table. For example, in the first group, the Atomic radius trend from smallest to greatest is: H (smallest) -- Li  -- Na -- K -- Rb -- Cs -- Fr (greatest).

Reason is, the increasing energy level (increases from n=1 to n=7) causes the shielding effect increase dramatically. And since the size of atom is mostly consist of electrons, more energy level is filled, the larger the atom will be.)

However, in the same ROW from left to right, the atomic size decreases as the number increases.   For example, Li #3, is larger than Flourine (F) with an atomic number 9 because they are on the same ROW.