Depends,
I apologize if I relate something you already know - I don't know your level of experience.
-When I shot Competitive Skeet, Most of us used 28" - 32" in bbls. [ Same for Trap, 5 stand, Sporting Clays, Live Pigeon...]
-Back then 21" and 23" bbl most often seen on "Bird Guns" - like Remington's English stocked Bird Gun. Guns carried more than shot for woodcock, quail and such.
- Today many Ranges do not allow bbls less than 26", 25", 24", or 23" . The individual Ranges sets a minimum bbl length to 1) discourage the short bbl tactical looking shotgun on the range. 2) Political pressures on Gun rights and such have caused many to "Play the PC Game" or to " Keep the Insurance Company Happy game".
Seeing a short bbl gun, with tactical stuff is bad PR. Scares the politicans, and gives fuel to the blissninnies whom hang out at ranges to "find fault".
So Check to see if YOUR club allows a shorter bbl...or ranges you might visit.
If you have a specific goal, type of hunting....etc., express it so we can better assist.
Pesonally - I have run a 100 straight many a time with a 18.5" bbl. For fun and because I wanted to - and could.
All 4 gauges I competed in - I used 28" bbls. I use 28" bbls for most everything. From HD, Doves, Ducks , Slugs....you name it. All my current SGs have 28' bbls. I do have a "special project" for a "special task" that does not apply to your questions. [Not wishing bad on someone, but if a long bbl gets err...,or dinged...
]
I ever get back into serious competition - I'm going 30" or 32" for sure.
Repetion becomes habit-habit becomes faith
I shoot from a low gun position, Many of us can take a 32" bbl , using low mount position, mount gun to face and fell a target / bird quicker than folks with shorter bbls.
Start with 25 Correct mounts of gun to face each day. Work up to a 100 a day. I assure you the efforts will pay dividends .
Get Brister's Book