F- & Mg+? ions are Isoelectronic ions with different sizes.

 F- with atomic no. 9 has 9+1= 10 electrons 

 Mg2+with atomic atomic no. 12 has 12-2= 10 electrons So both are isoelectronic species
 
Size of element increases down the group and decreases on moving left to right in a periodic table.

So size of  Mg2+> F-