How big does a place have to be to qualify as a wilderness reserve?

Meaning you don't need to have the same restrictions as zoo's etc..How big does a place have to be to qualify as a wilderness reserve?
A wilderness preserve is an area set aside for the preservation of animals and wilderness. It does not have to be a certain size but it has to be kept as a preserve forever once begun. There could be certain town or city ordinances in reguards to this but, there is not size limit on the area of the land.