Take a look at any of the cantilever barrels that are typically used for deer hunting and slug shooting. Common lengths are between 18.5 and 21 inches. But most of those shots are also taken on stationary game from a tree stand of sorts. They are usually equipped with rifle sights or a scope and probably harvest a ton of deer every year.
I think I would be more concerned with what load I was shooting. Just about any shotgun will do the trick. What I would look at is pattern density and accuracy out to certain distances to ensure whenever you pull the trigger, on whatever you're shooting, it puts them on the money maker.
If I was building a predator shotgun, I would go with a 24" or 26" barrel and test, test and test some more the chokes and loads that gave me the best patterns at the distances I think I will be shooting at.
