Is the boss of the chimpanzee community called the alpha male usually the smartest male or the biggest male?

It can be a little bit of both. Naturally, a strong male is going to oust a weaker male to become the alpha. As Frans de Waal has shown, there is quit a bit of politics that go with this title. Alphas form alliances with other males in the group, as well as ingratiating themselves with the females. Often times the position of alpha male is decided by the amount of support that one has from the community. Therefore, pure strength alone is not always a guarantee of becoming the "boss."