Pattern size depends a lot on your choke. I prefer IC for home defense and LM for most 3-gun stages. But inside your home with an IC choke, you will likely never go much over a 3" pattern or so...and that's in a BIG house.
I prefer reduced recoil, 8 pellet, 00 size (Remington Reduced Recoil or Speer "Lawman") buckshot for defensive use. I don't use birdshot for home defense due to potential lack of penetration on heavily clothed "suspects". Most people who use birdshot are trying to prevent over-penetration but I personally think that is a bad move. Don't get me wrong...birdshot will behave almost like a slug at short ranges and on lightly clothed targets but it loses it's punch very quickly when met with resistance. You could probably use heavier shot like #4 or smaller buckshot but once you get a load that will penetrate heavy clothing then it will probably penetrate sheetrock so the best option is to practice enough that you can keep your shots on target.
Measure the longest possible shot in your house. Go to the range and pattern test your shotgun with your choice of defensive ammo at that distance. It usually surprises people how tight the pattern is. It also usually surprises people that you actually have to be pretty precise with your shotgun. You can't just point in the general direction of the target and depend on the expanding pattern to do your job for you.
IDPA Member A00640
Founding Charter Member - Middle Tennessee Shooter's Club