I am writing a 2D game where one of the characters has some very particular requirements. This character is a body with no particular shape (similar to a fluid, but not so much), it has to be able to grow and shrink (as in actually growing, not just scaling), and it has to have collision detection (even if it's basic). Because of this requirements, it obviously can't be based on a sprite, so direct rendering of the shape should be the logical thing to do.

I assume this is no easy task, but I just couldn't find a good physics engine that covers these requirements (or at least no tutorial on how to do it; I particularly searched for Box2D tutorials).

Is there a way of doing this with Box2D, SDL, or any other physics or game engine out there?

If not, what's a good place to start? I am really clueless as far as soft-body physics are concerned.