The 64GB version of Surface Pro has 23GB of free storage out of the box while the larger 128GB version offers a bit more space at 83GB

From Engadget:

The 64GB $800 Microsoft Surface Pro comes only with 23GB of free space. that space for use. The larger 128GB version offers a bit more space at 83GB, but is still losing a massive 45GB of space to the full Windows 8 operating system and various included applications. Microsoft confirmed as much this afternoon, and issued the following the statement: "The 128 GB version of Surface Pro has 83 GB of free storage out of the box.

 128GB Surface Pro ships with 83GB free, 64GB has only 23GB