Many possible variants: set, fix, decide; number instead of size; military personnel instead of military forces. Or of course you could do it as a noun phrase if your prefer: "Determination of the size/number...." But the important point is that this doesn't refer to appointing or selecting personnel; it refers to deciding how many there should be from year to year.

This use of "fijar" and "fijación" is clarified by article 172 of the Peruvian constitution:

"Artículo 172º.- Fijación de efectivos y ascensos en las FF.AA. y P.N.P.
El número de efectivos de las Fuerzas Armadas y de la Policía Nacional se fija anualmente por el Poder Ejecutivo. Los recursos correspondientes son aprobados en la Ley de Presupuesto."
http://hukvida.org/leyes/constitucion.pdf

"The right of supreme command and the right to determine the size of military forces were both included in this sweeping executive power"
https://books.google.es/books?id=49bWRrBQP74C&pg=PA34&lpg=PA...