Ionic radius increases on going downwards in any group. In group 15, the last element SB and Bi have iconic radii lesser than the rest of its elements i.e N, P and As. This is because SB and Bi have completely filled d and f sub shells which have least shielding effects.Thus they do not guard the valence electrons feel almost whole nuclear charge. This results into attraction of valence electrons towards nucleus and hence size decreases.