The program its self plus the built-ins in the program folder take up 12.6 GB.
The data, however, is dependent upon how much you install such as third-party assets from other websites such as Jointed Rail, Railroad Mods, etc., DLC - downloadable content from N3V, and stuff from the the DLS.
With the ability to place the data on another drive, you can split up the load of everything that would be on a single drive.
Increasing the graphics levels or quality doesn't appear to increase the amount of disk space.
