I understand there are the extremests... some say Oscars need 6' 180 gal tanks... others say they'll be fine in a 3' tank...
Neither extreme is correct and the truth lies inthe balance... which is commonly the right answer in life...
But everyone says the people they do not agree with are the extremests. It's an easy way to discredit someone without actually giving credit to yourself.
It's a very well accepted fact that in healthy conditions Oscars grow an inch per month. Not every Oscar will grow at this rate and a couple may even grow faster. This is because every individual fish is an individual, which is also a very well known fact.
So if you buy a 3" Oscar, in 4 months you should have a 7" Oscar. Although this is only a little over double in length, the 7" Oscar will be several times larger than a 3" Oscar by mass. Also as the fish matures it's aggression matures.
For those who lack the personal experience with Oscars/Cichlids to understand the big picture of whats going to happen, I highly suggest you make conservative decisions to allow yourself to error on the side of safety, as opposed to unnecessarily punishing a fish for your mistake.
I've raised Oscars in a 300 galand I can say from first hand experience it made a big difference in the fish's attitude/behavior. But I do not fanatically state they "need" such large environments. I currently have 2 Full adult Oscars, each is in a (different) 75 gal with a couple smaller tankmates. Huge tanks are great, but not "needed". But this is no excuse to cram them in too small of a space either, as they do "need" room.
Again... the truth lies in the balance...