The majority of pigs will happily eat nettles.. although they can be like humans and some have there favorites flavours but when everything else is eaten they'll chomp and clean nettles, brambles, thistles etc etc
If you decide that you want to spray though don't forget the withdrawal period normally around about 2 weeks..
The first pigs I got I used Grazon to kill off the thistles, as I was worried about there noses, the only thing it did was make bare patches in the ground that allowed the pigs to start turning earth

As to 30 by 30 it's plenty of space in the dry just wont last as long in the wet