In order to take any advantage of the higher BC afforded by "extra-long" slugs, we would need to shoot them at a decent velocity. Say at least 950fps? Because of the relatively low pressures, trying to deliver enough air volume becomes problematic. Lets say that we wanted to shoot a 100gr .217 caliber slug at 950fps. That's 200fpe. With a 200cc plenum and 3000psi of air, that requires dumping it all into a 62" minimum length barrel in order to get the required energy to accelerate the air and projectile to 950fps.
I have not done a .217, but I have done a .257 and experimented with relatively long slugs. Best way to start is pick the maximum barrel length that you can live with, and then work backwards, with the other practicalities: such as available air volume and pressure, desired shot count, gun weight, etc. It becomes a game of compromises. For my purposes, "medium-length" slugs (72gr .257) turned out to be the best compromise for my end goals.