2 3/4"
-Patterning the shotgun at yardages will determine which load is best for one's gun.
-Reliable feeding and extraction has to happen for a shotgun (any firearm)to be an effective tool with loads it patterns best with.
It does not matter how long the shell is, or how many pellets, if the gun does not run.
-Quick effective hits with loads that feed and extract, and pattern well, are better achieved with less felt recoil.
Meaning shot recovery times are better.
-Ammo compatibility.
Logistics if you will.
2 3/4" shells will work in a shotgun with 2 3/4", 3", and 3 1/2" chambers.
If one only has a 2 3/4" chamber, and is tossed a longer shell, it will not work in that gun.
It would really be bad to have 3" shells, and an emergency happens, such as tornado, hurricane - any disaster, and the gun someone hands you to use, only takes 2 3/4" shells.
I am serious, folks have older guns, and folks still buy a great gun like Win 1200 for instance, it only takes 2 3/4" shells.
Keep it simple and break it down to the less is more game plan.
Same reason for having a spare box of plain vanilla , standard pressure.38spl LRN in case of emergency.
These will work in a old snub nose, more modern one, 4" gun and even in a .357 .