Roughly a hundred gigabytes. Probably more, really.
I seem to recall there being a physical USB key delivery pledge level, but it doesn't appear to be available anymore (problems with VAT because the game is on the stick meaning obnoxious taxes). Obviously anything short of bluray disks are out of the question. The prospect of 25 DVDs makes my heart a-quiver, and I sat and suffered through the six-CD installations of multiple games multiple times (UT2004 and HL2 if you must know).
I sympathize with your predicament my friend. Who knows? Perhaps closer to release date they'll have a solution of some kind. Maybe download the individual modules separately or something.