WEll depending on the hogs in the area your going to hunt I would go with 12 - 18", or simply just use 15" and be done with it. Most of the ones we shoot fall into the 80 - 150# class, with many hitting 200# and up.
Here is a very informative site on them,
And the link at the bottom has some of the ones we have taken this year and in the past. We got a nice 320# one the weekend after Thanksgiving, along with several more that were really nice.
Good luck on your hunt, and remember that their vitals are up front between the shoulders and not behind them. A good shot is just behind the front point of the shoulder and a little below midway down from the top. This generally drops then every time in their tracks and doesn't mess up hardly any meat. You will see what I am referring to in the pictures on the site above.