buckshot kills using a combination of penetration and number of hits. I would use nothing smaller than number 1 buck, but only if it patterns the best and keep the range to 30 yards or less. Bigger is better if your gun will pattern well with the ammo. Just keep in mind that a 00 buck pellet at 40 yards has the same energy as a .22 long rifle at the muzzle, hardly a deer bullet. I would only use buckshot if it was the only option allowed, go with slugs if they'll let you. Another great option is the Dixie Slug Companies "Tri-ball round", it is the largest commercially available buckshot load.
http://www.dixieslugs.com/products.html