This is the Jaffa take on this , there is a profile of rail outline placing of wide point and rocker that is a mal/ longboard, with variations of fin set up etc, a longboard had in its original configuration reached limitations that surfers around the world ( virtually California Australia and Hawaii then) were transcending in performance, shortening happened to get the boards to do what the surfers were able to do! Pocket history
The idea of parallel rails and other aspects of longboard design became outmoded and wouldn't fit a short template.
To cut to the chase with the question, pretty much the giant longboards of 10 11 12 feet are superceded, and are a rarity except among big wave paddling older guys.10 foot can be for a large heavyweight guy or a dedicated nose rider using the log weight to aid his cause.
9ft seems to be the norm and the weight and configurations within that length vary greatly but also the plan shape is always similar.
This shape can be shortened to IMO to about 8ft and still be a long board, mal, minimal below that I think they are long boards for children.
They might call them fun boards or other names but as they go shorter the rail lines begin to blend to Fish, Simmons, planing hull, and all sorts of oddities, some are repeating experiments that came and went in the short board evolution.
Some facts to compare, long = more volume easier to paddle as long as the nose rocker is not too great, harder to duck dive (bigger cork) especially for lightweight surfers, can be surfed in as big a surf as you wish, get slightly more problematical in steeper section take offs.
Short arc turns of short boards more difficult. Nose ride better especially with light weights.
Shorter lighter long boards beyond the oxymoron can be easier to chuck around in turn and things but if you have the technique and skills this translates up in length, 8ft is usually the minimum length allowed in longboard competition ( often has its own division)
Each board sold is a little exercise in commercialism, they all promise a wonderous quality of ride and increase in the rider's ability. All BS except for what you can get out of a board by your own abilities, ( some boards are dogs though)
The average rider won't notice a great deal of performance difference between the generics of the shortening of the McTavish boards except in paddle power!
Long winded as usual but hope it helps!
I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun!