The recommended stocking for pigs (if you're keeping them year round) is 6 per acre.
If you're going to breed you'll need a couple of acres (as a minimum) but if you're only raising a few for slaughter you can get by with a smaller area, say 20m x 30m. BUT with a smaller area you've got to make sure that the fencing is tip top, otherwise once they get bored and have dug up all the ground, they'll be looking for ways to escape - any weaknesses and they'll be off
The land should be free draining, not heavy clay - because although they like mud in the summer, longer term it's prone to causing arthritus. Permanently wet ground should be avoided for the same reason - but providing at least half the space is dry pigs will be okay. Established woodland is ideal as pigs, by nature, are great forragers and this also provides a bit of shade in the summer.
HTH
Karen x