Welcome, politically sober.
If you're concerned about overall length, consider a mossberg 500 or 590 with a folding stock. It's a pump "riot" type shotgun that takes 7 or 8 shells.
Basically, you have to make them, which is perfectly legal to do. Start with a "cruiser" model, which is inexpensive, and add a folding stock.
http://www.combatstocks.com/Shotgun_Stocks_Folding.cfm
Personally, I like the Choat type, although the ATI universal is fine too.
The reason you want this stock is A) pistol grip shotguns are NOT easy to handle. It's much better to shoot from a shoulder mounted position. B) If for whatever reason you DO need to fold it up and shoot from the hip, the extra weight across the top makes it much more manageable.
Also, since you're a new member, I am going to presume that you're more or less a nonshooter (ie: new or occassional), looking for a "just in case" house gun, and so I give you my current advise for such folk:
Give serious consideration to a semi-auto pistol caliber carbine, such as the kel-tec sub 2000, Marlin Camp 9 or .45, Ruger PCxxx series, or Berretta Storm, to name a few.
The reason I recommend them is because being long arms, they don't require the level of mastery that a handgun demands (and avoids the various complications that some handgun averse jurisdictions throw at citizens). In fact, they make it hard to miss at home defense ranges. They're not as powerful as a full blown rifle, and this makes followup shots faster and easier. They also don't recoil as much, which is often a problem for the new and occassional shooter.
If you are familiar and comfortable with shotguns, the following are generally considered the best of the defensive shotgun family, listed in the generally accepted order of preference:
Remington 870P (militarized 870)
Mossberg 590 (militarized 500)
Remington 870
Mossberg 500
Winchester 1300 "defender" configuration
The militarized models are essentially the same as their civilian counterparts. Some of the parts that are rendered in polymer in the civilian models are beefed up in the militarized models, and are therefore more durable. The civilian models are considered sufficiently durable and reliable as to last for generations, and you would be well served with either the Rem or the Mossy. The jury is out on the Winchester.
All these are pump shotguns with extended mags and 18-20 inch barrels (the defining features of a "riot gun"), carrying 5-8 shells, depending on the model. The mossberg has a tang safety, which lefties like me consider to be less hostile than a cross bolt safety. There are some other subtle differences.