If you are buying a Microsoft Surface Pro from 9th February 2013, I suggest that you buy the highest capacity available. Based on a report from Softpedia, the unofficial space that Surface Pro will take up is around 45GB.
That means that if you were to buy a 64GB Microsoft Surface Pro, the remaining storage is just 19GB. That is, in my opinion, pathetic. And if you buy a 128GB version, you are left with 83GB of usable storage.
Softpedia got this information from Microsoft based on questions that they had asked.
The company, however, told us in a statement that the 128 GB version of the Surface with Windows 8 Pro offers users 83 GB of available space, which means that the operating system and the pre-installed software eat up approximately 45 GB.
So, I suggest that you read the storage FAQ first before you buy the Microsoft Surface Pro when it is launched.