For me I guess 22" and up.
Why the heck would it be any less?
Crap the good old 24" slapped a rifle on it! or just me once again.
I see your point, and concur, with minor reservations.
A carbine is less than 26 inches in length.
Twisty barrel longer than a pistol, I reckon.
If it is longer than 16", it is a rifle. Shorter than 16, it is a pistol.
Other than lever actions there has never been a clear definition. Call it what you want.
I have them with barrels from 18.5" to 24".
The shorter ones are handier to tote in tight places, but I prefer the steadiness of a 24" barreled sporter.
If I were told I could have just one, it would be with a 22 to 24 inch barrel.
The Wacky Weed must be in abundance this season...