The space would depend on a) whereabouts you are in the country, and b) whether you want to maintain grass.
HH is in Scotand where it is wet and cold for much of the year (and beautiful!). Also HH uses her Kune Kune's as grazing. So her recommendation of an acre is perfectly valid for her.
Depending on what sort of weather you have, if you are happy with them turning it to earth, and to some mud in the winter, then a pair of kune Kunes in 1/4 acre should be ok.
and yes it would be a pair, and they need company of their own kind.
DON'T go near micro pigs - get Kune Kune's they are cute, pretty, and hardy, and great as pets.