To be competitive you need a shotgun that will hold at least 8 shells. The key to shooting a shotgun in competition is reloading. If you plan to shoot in the Limited division you will be restricted to 8 rounds in the tube, iron sights and no porting. Shot shell length is usually limited to no shorter than 2 3/4 in. The trick of using 2 inch shells to get more in the tube is illegal at most IPSC 3-gun matches.
For ammo I generally use #6 shot on steel and reduced recoil slugs on paper targets.
You don't want a long barrel. I have an 18 inch Benelli M1 S90 with screw-in chokes that works very well going though doors, shooting through ports, around corners, etc. I know some guys with mag tubes that will hold 18 and 19 shot shells in their "space guns" competing in Open class. They may not have to reload, but they might as well be shooting a musket when it comes to manuverability.
As far as reloading during a COF with a shotgun there are two methods that I see most competitors using with any success. Tec loaders and shell holders on the belt. Tec Loaders are quick, but require lots of practice especially on the move. Shell holders on the belt hold 6 shells lengthwise. With practice you can grab 2 or 3 at a time from the shell holder to load into the gun. Depending on the COF I will carry 2 or 3 shell holders on my belt.