How would you account for the fact that the F- ion is smaller than the O2- ion?

F ion is basically Fluoride ion, and O2 is oxygen ion. The fact that fluoride ion is smaller than Oxygen ion is because fluoride has an extra proton in the nucleus area, which holds the electrons slightly closer, and hence its size is slightly smaller than oxygen.

Generally, as we see from left to right in the periodic table, the atomic radius decreases, and as you move from top to bottom, the radius increases as an energy shell is added.