What happens to the size of the associated ions: e.g. Na+->Mg2+->Al3+?

Answer 1

The size is decreasing as you go across that list. A rough rule of thumb is "The more positive the charge, the smaller the ion." (The reverse is true, too...the more negative the charge, the larger the ion.)

Answer 2

These ions are isoelectronic - they have the same number of electrons. However, as you move across the periodic table, the number of protons increases.
The ability of the nucleus to pull electrons towards itself increases, so the electrons are closer to the nucleus. This makes the atomic radii decrease in the order Na+>Mg2+>Al3+.