Several years ago, about the time we began the Ceph Care forum, it was believed that a 30 gallon tank was plenty large enough for a bimac. I believe this recommendation originally came from research labs with flow through systems (seawater constantly flowing through the tank, therefore no problem with filtration) and a mostly bare tank, offering the maximum space for the octo.
But, after watching our first generation of bimacs, it turned out that a fair percentage that survived 10 months with their owners were large - large enough to be cramped in a 30 gallon and to overwhelm the filtration. We also learned more about how bimacs swim and jet around and how much space they need to live a somewhat natural life. One of the most successful bimac keepers I've talked to (17 consecutive bimacs) noted that the larger tank he used (90 gallons, I believe) allowed you to see how the octo set out everyday to check certain points in the tank. Octos in larger tanks take advantage of the room in the tank as soon as they grow up a bit.
So out of this came the recommendation for a 50 gallon tank as a minimum. A 30 gallon tank is better suited to a dwarf octo or smaller species.
There have been people who've kept a bimac in a 30 gallon successfully - and some bimacs have stayed small, even in big tanks. And some stay small because they don't live as long, sad fact.
A long tank is better than a tall tank because it gives your bimac more room for swimming and more bottom to explore. If you have a choice, go for a longer rectangular tank and I would strongly recommend a 50 gallon or more.