(DPris, if that's you, I read your 20 gauge shotgun review in one of my subscribed-to magazines...nice work as usual!)
reference:
http://www.remington.com/products/firearms/shotguns/model_1100/model_1100_sporting_20.asp
You might consider a Remington 1100, however, a shorter barrel will likely be desired, which will run ~$200, delivered, from somewhere like:
www.brownells.com
There's even a 20 gauge mag extension at:
http://www.wilsoncombat.com/sa_mag_extensions.asp
With all that said, I don't agree with the 20 gauge semi-auto for you. I've seen our 5' tall female deputies qualifying with our 12 gauge pump shotguns with no problems, given instruction, practice, & training.
My major concern is the lack of ammo variety for the 20 gauge. I see no 00 buck, which I prefer, although there is a variety of 20 gauge slugs on the market. There are also 20 pellet #3 buckshot rounds, however.
12 gauge is the best way to go for home-defense shotgunning, especially if you haven't yet bought the gun.
My advice: Consider a 12 gauge, of which there are many, or an AR-15 variant.