Tonearms longer than 12 inches

Long ago, and not often remembered is that 16 inch acetate disks were in use for recording use, so you needed a 16 inch arm. The disc cutter TT and arm could, of course, play anything below that. I had such a unit when I got interested in doing direct to disc recording. I did not have any SOTA electronics at that time so I can't comment on the quality of the sound produced. My 'sound' memory has lost all remembrance. Were talking late fifties. I am sure there is 16 inch units out there and they would be fun to play with.

Who knows? Everything in analog comes back sooner or later (normally worse and super expensive). At the moment we see the comeback from
12" Arms...3 years ago I can remember that the audiophile community wrote, 9" is the best ever :-)
SAEC made a very long arm in the 80's but in a way, 12" was the way most manufacturers choose (in a time when real engineers thought about that and they had knowledge about distortions)
Generally it will depend how something is made and what design Solutions are realized but there are 2 problems:
The majority of tables can't even use a 12" Arm (the only way is a Design like Kuzma 4P or tables with separate Armboards) and the Alignment Systems for it (good ones, we have today endless of them and most users wrote, when they switched from this to that, the improvement was huge, so I guess :-), the most of them are not useable generally for all Arms, even when the marketing suggests it).

I think this is true: The longer the arm, the more accurate one must be in mounting it in order to take advantage of the theoretically superior groove alignment. We are talking about tolerances well below 1.0 mm.

In addition, the longer the arm, the higher the effective mass for a given material, which must also be taken into consideration.

Women audiophiles prefer the longer ones:)