The info I got was that the 71 is tank measured at 72 liters. As for sizing and multi fins, I think that only twin fin boards can truly be "oversized" for real wave sailing but pretty much all multi fins handle more wind in straight line sailing than a comparable single fin .
In other words, for performance wave sailing I don't think tri fin or a quad really should be sailed bigger (wider) than a comparable single. There are some brands whose quads are really thick and then you will off course get more volume per width and hence you can ride a bigger volume board, but in my opinion, adding volume along the center like that will actually create a significantly more unstable board when slogging (and that kind of volume only help at absolute standstill when you in fact sink). Starboard generally use rather flat decks and low volume relative the width.
Talking about the Quad71, its in fact pretty very similar to the 07 and 09 EVO 70. All those boards are excellent slogger relative their volume and they stay super stable and easy in semi-submarin mode (the only better board at this was the 09 ET70 which at a real 72 liter was just unbelievable in super light wind).