From Microsoft's own website...
Surface with Windows RT comes with either 32 GB or 64 GB of internal storage space (hard disk space). However, not all of this space is available for your music, photos, videos, and other content.
How much storage space do I have?
Here’s how much hard disk space you have available out of the box:
The 32 GB version has approximately 16 GB free hard disk space.
The 64 GB version has approximately 46 GB free hard disk space.
Well that's a fail and a half!