THOUSANDS of deer have been killed in the Southern US with buckshot. Ive seen them killed with everything from #4 buck to 000. Rule of thumb was to use the biggest pellet that your gun would pattern in a paper plate at 30-40 yds. That happened to be Winchester Super X #1 buck out of my Remington 1100 and Beretta 390 shotguns. In my guns a modified choke worked best. From the late 70's until present, that combo has accounted for dozens of dead whitetails. If possible I tried to shoot them in the head and neck, but shots to the vitals were equally lethal. Some pellets passed completely through and some didnt.
Long story shortened, go buy a few boxes of buckshot, see what patterns best out of your gun then have faith that reguardless of shot size, it will take the fight out of any would be home invader.