From my experience, I'd recommend this:
1 - build the pen as large as you need and surround it with a fence. place crafting stations within the pen so, that their bounderies cover the entire area.
2- Create a buffer zone of 8-10 meters (4-5 2x2 floor planks) out from your pen - This should be far enough that the log trolls cannot bludgeon your boars to death
3 - dig a trench just outside your buffer zone, trapping enemies. So when, enemies are near and the pigs run around scared, your boars will be safe within their pen.