so my boar pen usually covers the area of 1 crafting table, i level it out as much as possible and build a gate airlock (for new tames and to section out boars when needed.)

around this area, i build crafting bench huts to keep spawns from coming around as often. these are often far enough away i also put torches or some other form of fire at them. this way if a boar gets out it'll run away from it and make noise. (usually back to the pen.)

and at the boarder of THAT ring of benches, i do wooden stake pain walls. with short wall behind them, just something to keep myself from bumping into them.

a pen this size can contain a good amount of boars and if you wish, make a great sized crop area as well. and the pain wall maintains itself as long as it kills greylings and the wood is collected.