I think the answer to that is 'huge'. Here's an image of the inside of the new Macbook:
Beautifully put together, but if I'm not mistaken, almost half the internal space is taken up by the custom battery cells. That (and the removal of the hard drive and optical disk) is what it takes to keep this machine running for seven hours without a recharge.
Does make you think.
That many batteries can't come cheap, either....