It is the union of different states, which follow some common rules under a common leadership, and keeping also a bit of independence in order to avoid better the possible neighboring corruption ; more the federation is composed of a high number of different states, more the cohesiveness has to be good in order to ensure an equivalent functioning in all the parts, especially if these states have a rather high population and area, because the listlessness in the changes is a lot more important, because some parts of the population resist in bulk to some other parts which like in the areas close to the border are more open to the foreign influences ; thus it is preferable at least that there are not any tension between the populations of the different states. On the top of that in order that it functions well it is better that the constitutions of the different states are applied at least to some governments which have the same basis, like with some republics ; if not certain important splits that it implies can engender some resistances which are the ones from the transition between a manner to govern and an other one ; this is important in order that the different parts can use the same laws according to some interpretations the closest possible.
Republics having to maintain a moral standard sufficient in order to orient well the decisions of the people, they must have a cohesiveness the strongest possible, thus their size is preferable when it is restricted, according to what they have to unite in order to resist in the case of an attack of a (some) state(s) less concerned by its (their) cohesion(s) for example ; but it is true that the limitation of the territory seems strange if we consider the current republics, and this because of some things which did correct this problem like transports and media, if there is an effort in order to minimize the possibility to spread vices. But it is more difficult to make cohabit some states like some monarchies which let easily vice spread and are thus aggressive, with some others which have to maintain virtue (so wisdom) hence a rather peaceful atmosphere. But if a union has to be done between some too much different states and then too much independent ones, there is all the same the possibility to make turn the government between the different members (see J.J. Rousseau, “The Social Contract”, book 3, chapter 13).