I believe the manufactureres determined whether or not an engine was a big block or not. Example: Pontiac's 350, 400 and 455 are all based on the same block. It is a large block with a large deck height, very similar is size to a Chevrolet 396 (Big Block). However, they are most often called Wide Block Pontiacs. Never heard the term Big Block Pontiac, but by all rights, it could be called that.
I also tend to see big torque numbers at lower RPMs in Big Blocks but they often don't rev quite as high. But that depends on how they're built.
Fact of the matter being: I don't believe there is a set dimension for big block vs. small block. Check it with your eyes, and ask someone if you're wondering. Pretty much every American OHV V-8 has already been catergorized into one or the other.