O electronic affinity

The 1st electron affinity of oxygen is exothermic, but the radius of O- (ionic radius 0.176 nm) is greater (surely NOT smaller) than that of O atom (atomic radius 0.06 nm).

The 1st electron affinity of oxygen:
O(g) + e- → O-(g)
The attraction between the incoming electron and the nucleus is exothermic, while the repulsion between the incoming electron and the original electrons are endothermic. Since the attraction between the incoming electron and the nucleus outweighs the repulsion between the incoming electron and the original electrons, the 1st electron affinity of oxygen is thus exothermic.

The radius of O- ion is greater than that of O atom. This is because more electrons share the nuclear attractions in O- electron than in the O atom, and O- ion has a greater repulsion between electrons as O- ion has more electrons than O atom.