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Cl^2- because of the bigger proton count. Since they are isoelectronic (where they both achieve the noble gas configuration), you should look at which has more protons to attract the negatively charged electrons. This will, consequently, lead to a smaller radius.
Just to add on to the above responses, as you go from the bottom left to the top right of the periodic table, the atomic radius generally gets smaller because there are less energy levels as you go up and more protons as you go to the right. More protons means that there is a stronger positive attraction, pulling the electrons closer to the nucleus, and thus a smaller radius.
In class it was discussed that anions are always larger than their parents ions (and cations are smaller). Phosphorus is the the largest anion now with the addition of 3 electrons to shield the attraction by the nucleus, whereas the chloride anion only has one extra election, providing not as much shielding and thus the nucleus has a greater attraction to pull them in for the smallest ionic radius out of the three. But the number of protons is also significant, contributing to the attraction of electrons to the nucleus for the factor of atomic radius as well!
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