some know-it-alls aren't always wrong either. I wouldn't classify the Viper as a "big block". More like a longer small-block, since the block itself isn't nearly as huge/heavy as the old musclecar bigblocks.
Cubes do not define big block vs small block, those are descriptions from physical dimensions, which also dictated the size of the heads. Viper engines are just stretched longitudinally due to the extra two cylinders, but their features are similar to any other small block.