A Question about the asteroid belt?

I've read up alot space and what's out there, Discovery, Wiki, etc. I had a question about the asteroid belt between Mars and Jupiter in regards to the theory that its the remnants of a broken up planet or a planet that never formed. What would be the approximate volume and m of the planet they would've formed in relation to earth, and also could they now be in the process of forming into that planet, maybe with Ceres being the start of a core?