Back when pumps & semi's were the most popular for Skeet, the 26" barrel was the accepted standard
and 30" for Trap guns.
I've shot Skeet reasonably well with a shortened 20" SxS and short pumps and semis
(the Remington straight stocked 870 & 1100 "Upland Specials" IIRC, 21" or 23"), but the
'swing' and 'balance' of such guns leaves a lot to be desired for targets.
A 26" or 28" pump or semi is considered by many to be of versatile length for both
hunting & clay target shooting, and generally, offers more to overall balance and
a smooth,steady swing than will a shorter barrel.
In an O/U or SxS, either a 28" or 30" is more customary, and some (including me) prefer
32" barrels for Sporting Clays, 5-Stand and Trap ... and I don't find it cumbersome for
Skeet.
Overall, for targets, if given the choice, I prefer a longer rather than shorter barrel.