The Creeper is much like the Corruption, and could easily be an earlier or more violent form of it. The Creeper engulfs anything it touches, Necromorph or human. It will absorb any organic matter in order to make itself bigger.
It's total mass and size are unknown, but are probably close to whatever the mass and size are of the thing it is infesting.
A possible equation for its mass would be Mass= Infested Area - Places Unable to Grow (insides of solid walls, water, etc.) - Empty Space + Mass of Objects Consumed + Area Covered.
A possible equation for its size would be Size= Infested Area - Places Unable to Grow (see Mass Equation) - Empty Space + Depth.