This video shows you the two main problems you need to know for this topic. The main one is knowing how to use the periodic table to figure out the relative size of one atom versus another. The other is comparing the size of an ion to the neutral atom of the same element.
If you do not have an account, you should get one, because it is awesome! You can save a playlist for each test or each chapter, and save your "greatest hits" into a "watch right before the final" list (not that we recommend cramming, but when in Rome...)