Remington 870 Youth in 20 ga. Not too short for large folks, size and recoil easily handled by smaller statured folks, and even 7/8oz of shot is going to seriously mess up a bad guy's day.
Even with an improved choke, pattern spread is going to be less than 1' diameter at distances you would find inside most homes.
Ed
I've got a NIB Mossberg 500 bantam youth 20 ga. I'd sell if a fella likes this line of thinking.
Hard to go wrong with that, either!
IMHO, the guys on here need to remember that it may be a smaller statured person in their house that is called on to defend and a shotgun (or any long gun for that matter) which is too long/heavy/complicated will not be doing them any favors.
Ed