You should not do a 20 pound turkey in a smokin-it. Because the box temperature doesn't get above 250 (on an analog unit), the bird will sit too long - more than 4 hours - in the danger zone for bacteria growth (40 - 140 degrees F). The biggest suggested for these units is 12 - 14 pounds.
Interestingly, if you cut it in half, it is like having 2 10 pound pieces of meat, which is fine. You will find more on this in the Poultry section of this forum.