Not smaller than #1 Buckshot, with 0 or 00 preferred. 00 Buckshot it probably be absolute best choice.
Some folks will spout crap such as "You don't have to aim a shotgun, just point in the general direction and you can't miss," or "Use #8 birdshot for home defense." Horse manure on both of those pieces of ill-advise.
At inside the house ranges, the spread of a shotgun charge is going to be no more than four or perhaps five inches. It is easy to miss with that small a pattern. Point carefully or you will endanger innocent bystanders or neighbors.
As for the #8 shot advise, leave the birdshot for birds. At less than 2 grains in weight, and a velocity of around 1300 fps, a piece of birdshot won't be able to penetrate to a badguy's vital organs, resulting in a nasty surface wound that might not be debilitating.
For good advise on shotgun ammunition, check out the Box of Truth's articles at:
http://www.theboxotruth.com/docs/bot3.ht…