Cobalt nanocrystals are produced in cobalt(II) bis(2-ethylhexyl)sulfosuccinate (CO(AOT)(2)) reverse micelles. It is found that their size distribution is related to the volume of reducing agent added to the micellar system. At a low volume of reducing agent, the micelles play the role of nanoreactors in which take place the nucleation and growth of cobalt. In the supersaturate regime, that is, at a high volume of reducing agent, micelles are destroyed. In the first case, the nanocrystal size polydispersity is about 29%, while in the latter case, it can decrease to 8%. This low size polydispersity has to be related on one hand to the excess of the reducing agent that modifies the nucleation and growth processes and on the other hand to the structural and chemical evolutions of the micellar system. To obtain well-defined two-dimensional superlattices, the size distribution needs to be less than 13%.