What a great Gift! I'm assuming you have already miked the bore to verify its size. If so, 14 gauge is correct. Felt wads in that size might be scarce to locate tho'.
Should nothing be immediately available for your bore size, you could: 1) substitute card wads, 2) order a wad punch to make felt wads, 3) use available standbys, such as tow, wasp/hornets nest, soft leaves, or possibly even cornmeal/polenta for the O P wadding. Somehow, they all seem to work. Congrats on your new smoker - have fun and enjoy the journey!