I always found it depended on the factory. I don't believe there has ever been an exact definition as far as dimensions across the board. Chrysler's BB motors are broken down into B and RB classes. RB standing for raised block, (taller deck height) were 426-440's B's were 361/383/400 ci. All had dist. in the front.

I think it started back in the 50's when car companies started offering larger optional motors then what was always available in the past.

My definition of a small block is whatever the factory considerd it.