I once had the opportunity (or misfortune) to witness the results of a shotgun shooting, the shootee taking a load of #8 shot (1 1/8oz) at about 6 feet in the upper chest from a sawed-off single barrel 12; it bored a half-dollar sized hole clean thru the chest wall and out the back! Based on my experience of shooting thousands of shotshells over the years, it is my opinion that in a situation where a shot would be taken across a room, say a distance of 12-16 feet, I don't think it would make a whole lot of difference what size shot the shell contained; a solid hit would inflict a massive wound. Should the drug crazed fellon be able to get up after that first shot; then shoot him again!
As for me personally, in the gosh-awful event that I should ever be unfortunate enough to be blasted at relatively close range with a scattergun (far enought away for the pattern to start opening up), I would hope that it would be with buck-shot; and that I would be humanely dispatched. Can you imagine someone trying to find/pick several hundred pellets out of your carcass in the event you survived? But for those who doubt the ability of 4 & 6 shot to kill humans at distances beyond 20/25 yards, let me remind you that such loads routinely kill a few unfortunate hunters every year during turkey season.
As to home defense, obviously a short barreled pump or semi would be a solid home defense weapon; but even a compact shotgun can be a bit unwieldy for a lady, a fact even more true in a panicked situation. Given that fact, I selected a Charter Arms 44 Special revolver as my wife's home defense weapon. It has a 3" barrel, no hammer spur (to avoid hanging on clothing, etc), is compact, is relatively light, and has no confusing safety for her to worry about. It is loaded with Winchester hollow-points for maximum shock; and all she has to remember is to just pull the trigger.
Finally for those who prefere a shotgun for home defense, my shot size of choice would be #4 buck. Depending on shell length and load, 18 or more .22 caliber sized pellets are delivered on the target (43 in a 3 1/2" 10-bore!). I have witnessed the effects of #4 buck loads on deer, and it is devastating. Additionally, #4 buck will pattern much better than 00 buck in most shotguns.
My hope would be that none of us ever have to face down an intruder, but it sure sounds like you guys are ready.