How big is the Portal 2 download from Steam?

Solution 1:

The system requirements list 7.6 GB for Portal 2. If you already have a Source game the size could be much smaller since the Source engine compromises a large part of the download.

Solution 2:

The amount of data that needs to be downloaded is different than the amount of disk space the game uses once installed. As reported by the installer screen before installing Portal 2 the disk space required during the installation is 11.6GB:

However once you start the download process, the amount of data required is significantly less at 6.6GB. Additionally, I have tested that this download size is not affected if you already have the Source SDK or other Source based game(s) installed.