Dirt 3 and Showdown Size?

More than one byte. That's all I know.

Flatout 3 is 11,045 MB (~a little less than 11GB).

Dirt3 = 11GB
Dirt Showdown = 9GB

Just checked my Dirt3 file. it says 11.5Gb

Why don't you look on steam? All games are required to list the install size of their game on the bottom of each game's page in the store... Not to mention, GMG also does the exact same thing.

Here it clearly says "Hard Drive: 15 GB HD space", and the same for Dirt 3. I can tell you that my install for Dirt 3 is ~12GB, and the 15GB number is probably leaving room for the DLCs, especially the extra tracks.