How to determine relative size of polyatomic ions?

How would you determine the size of a polyatomic ion?


For example when determining which ion is bigger, $\ce{NH2-}$ or $\ce{I-}$, what would you use to determine the size difference?


I know that $\ce{I-}$ would be much smaller than $\ce{I2-}$ (since the $\ce{e-}$ are not held as tightly in the $\ce{I2-}$ causing the repulsion to increase)


But what basis do we use to determine size of a nonelemental atom like the polyatomic ion or a more complex molecule for that matter?


