What is the correct order of atomic radius of Li, Na, Be and O?

Atomic radius increases down a group and decreases across a period. Na (Period 3) is larger than Li (Period 2) since both are alkali metals but Na is lower in the group. Within Period 2, Li > Be > O because atomic radius decreases left to right due to increasing nuclear charge with electrons in the same shell. Hence the correct order is Na > Li > Be > O. Typical approximate covalent radii: Na ≈ 154 pm, Li ≈ 128 pm, Be ≈ 96 pm, O ≈ 66 pm.