I have heard the game will be around 15gb if it is similar size to the press release. As for the retail bonus, you have to ask yourself how much you care about that stuff. I have zero interest in "bonuses" that are not game content so that is pretty much worthless to me.
It requires 20GB space to download but it will be less than 9GB.
Over at the 2k forums, a rep had this to say:
"The 20GB in the system requirements is the amount you should have free when you install. It does not mean the download will be that large.
Remember, the game fits (compressed) on a single dual layer DVD, which is 8.6GB. So you can be fairly certain the download won't be more than that!
I'm looking in to pre-load"
He later confirms pre-loading.
It all can be found here!