Probably already been answered, but will it be an automatic download? I hope not. I have a data cap and I only have 12gb more until I got over that. Got 8 days left till the new cycle starts.
If you reserved it it'll be an automatic download. Roughly 3GB in size.
If you're running Windows 8.1 though and you're connected via wi-fi, you can set your network as a metered network to prevent the automatic download.