There are a number of issues that influence tank size. However, they boil down to breakage, maintaining water quality, and activity.
Small tanks tend to be glass and have thin glass. A 6 inch O. scyllarus could easily break a standard 10 gal glass tank. Glass tanks over 25 or 30 gal are made of thicker glass which are probably O.s proof. Acrylic tanks are safe. You can also line smaller glass tanks with acrylic to make them safer.
Perfect water quality is great. The trick is to keep it that way. The tendency for large O.s to bury their molt skins and surplus food can lead to a nasty surprise spike in ammonia. Small tanks leave very little margin for error. Also, most small tanks do not have skimmers to reduce problems from decomposition.
O.s are active animals that do best when they have room to roam. Animals that sit day after day in a pvc pipe in a small aquarium usually have more problems molting, etc.
That said, I would not want a 6 inch O.s in anything smaller than a 30 gal, but that does not mean that it can't or is not done.
Roy