You don't need a sword to kill a pig. Once the dogs catch the pig by the ears/snout/legs, the pig is more or less immobile and you can walk up and grab the back legs "wheelbarrow" style. Then you twist the back legs and push with your front foot to flip the hog onto its side. At this point there are dogs everywhere jockeying for position and it can be dangerous to have a spear in the mix. Once the pig is down on the ground, you unsheath your knife, place the point low on the chest right in the soft arm pit. Don't stab the pig, but rather just push on the back of the knife. If you put your blade vertically it should go between the ribs like butter. Then the knife is thursted in and out at varying angles to ensure sufficient damage. When you hit the heart you will know. Blood will gush out onto your hand and knife handle. Then you sheath the knife and hold the pig down until it expires. It is relatively safe for the hunter to do this when done correctly.
I have been hunting hogs since high school and have seen at least a hundred killed by knife. My friends have used every knife under the sun. I'd say as long as the thing has a secure grip that will keep your hand from sliding up the blade, and a minimum of a 6" blade it should be fine. Anything over 10" you run the danger of sticking all the way through the pig and cutting your own dog. it HAS happened. I think the ideal length would be between 7: and 8.5". I would prefer around 7.5". It also should be pointy enough to penetrate and SHARP.
My friends have used Kabars, different types of bayonets, RTAKII, Buck 119s, modified machetes, schrade deerslayers, generic military kabar type knives, benchmade skinners, screwdrivers, just to name a few...
I've even seen a small 2.5" blade pocket knife do the trick (jugular), as well as a normal ontario machete. The machete was used to slice through the pad, then with the existing slice it was easily stuck into the heart area.