Shotgun Info

[ksig1869]

Hey I've been doing my research for a shotgun. ( Haven't decided between 1911, shotgun, or rifle). Im doing my research in stages so I can make a decision at the end. And I was hoping that you guys can correct me on what Ive learned so far.

Shoots Shotgun shells but can shoot slugs but has to be through a rifled barrel.(?)

Choke determines the spread of the shot and Modified Choke is recommended to for newbies. (?)

Barrel length determines the accuracy and effectiveness of shots. Shorter barrels shot expands quicker, longer barrels shot more concentrated longer.(?) I was leaning towards 20" or less.

Gauge is how big the barrel is. Lower number= Bigger Barrel= More Shot
I was leaning towards 20 gauge.

The one big thing that I have been caught up on is the different shotgun shell sizes. 3" 2 3/4". What does this mean? Is each shotgun sized differently? If so, can you get a 3" and shoot smaller shells?

This would be solely for Home Defense with occasional shooting when I can find a place and time to shoot it. Sorry if this rambles. Any help would be great, especially if Im missing or misunderstanding something.

I was looking at the Mossberg 500, Remington 870 and Ithaca 37. I favor the Ithaca because I just like the bottom ejection and just like the way it looks. Oh yea Im looking at pump-action. Thanks