Hi Chris,
As far as I'm aware there's not a minimum set area to class them as 'free range'. If you're raising them for meat, it's best not to give them too large an area (or else they burn off all that lovely food you'll be buying) The recommendations for adult pigs is 5-6 per acre, if it's weaners raised to pork weight you're thinking of a quarter of an acre should be more than enough for 3 or 4. As for arcs - everyone's got their preferences and the range available is endless - plastic, wood, metal - insulated or not, floored or not and a lot depends on the land and type of pig. For example if you were keeping large white pigs outside over winter on wet ground I'd say go for an insulated, floored arc to keep them warm and dry. But if you were keeping Tamworths on dry ground they probably wouldn't need the insulation or floor. It's a kinda 'how long is a piece of string' topic

If it helps, my Kune Kunes live in an unused diesel tank with a door cut in the front and plenty of straw for bedding and my current 'meat' pigs are in an arc made of old pallets (with the gaps filled with lengths of timber) and the roof off an old VW camper. I have double skinned the pallets so that in the winter I can stuff straw down the gaps to keep them really toasty

Good luck !
Karen