Atom size general trend:
1) Atom size (Atomic radius) increases from up to down along each 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.)
2) Atom size (Atomic radius) decreases from left to right along each period of the periodic table. For example, in the second period, the atomic radius of Nitrogen (N) is smaller than that of Lithium (Li).
Reason is, the increasing nuclear charge (N has 7 proton while Li has 3 proton) pulled back the electrons with a greater power, making the atomic size of nitrogen smaller. Meanwhile, all the elements including N and Li in the second period has the same number of electron energy level (n=2), so shielding effect is the same and does not affect the judgement.