Build it big the first time and you won't have to go back, lol.
60ft diameter is the minimum I'd build. Need an entrance chute that has 2 to 3 sections (chutes)....behind that a separation pen or two...those pens lead to your main holding pen. You'll also need an exit chute/pen...preferably not much bigger than a big boar...don't want a dog to want to get in there with the pig. That exit chute should be attached to one of the separation pens.
Just an idea anyway
