There isn't really a minimum size (other than for indoor housing - which is probably smaller than most folk imagine

)
Stocking density for pigs is advised at 6 per acre, but if it's a couple of weaners for pork you can do it with a bit less.
I find that the smaller an area is the quicker it gets trashed (especially with the wet weather) and the type of ground you have will affect it too - if you have wet ground you might need two pens to work in rotation.
I wouldn't try it with less than about 20m x 20m, but the more space you can give them the better IMHO
HTH
Karen
