I've seen some pens 10 x 20 dry lot for big buck... to several small Nigie or pygmy bucks in the same size. Depends on whether you want them to browse and eat off the land or dry lot.
Just keep in mind your region and breeds of goats... too small of an area and you will end up with parasites if you don't basically do a dry lot feeding ...meaning they don't eat off the ground but have hay and feed provided up off the ground.