There’s been a lot of talks going on about exactly how much free space will be available for the users after the initial Operating System and Default apps in the Surface RT/Pro devices.Thankfully,Microsoft has put up a detailed Page which discusses the User available storage space on both Surface RT as well as the Surface Pro devices.

The Surface RT which comes in 2 variants the 32GB and the 64 GB versions.While the 32GB version of Surface RT features 16GB of User available space the 64GB version has around 45GB User available space.
Here’s the breakdown of the Surface RT’s Storage
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Surface with Window RT (decimal system)
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32 GB
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64 GB
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Total storage as reported by Windows (binary system)
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29*
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58*
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Storage space reserved for Windows recovery tools
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-5
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-5
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Available storage reported by File Explorer
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24
|
53
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Windows RT, Office Home & Student 2013 RT Preview1and built-in apps
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-8
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-8
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Available storage reported by File Explorer
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16
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45
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So roughly around 29GB actual space (as per the binary system) is available in the 32GB version,with around 8GB space taken up by the operating system,Office suite and built in apps,while around 5GB of space is reserved for the Windows Recovery Tools,giving users 16GB of storage space.Similarly with the Surface RT 64GB version, around 58 GB actual space(as per the binary system) with same amount of storage space accumulated by the Operating System and the Windows Recovery Tools,giving the user 45GB to play with.
Talking about the newly launched Surface Pro,which runs Windows 8 Pro operating system and sports 64GB and 128GB as storage options.The 64GB variant has around 29GB of user available storage while the 128GB version has around 89GB of user available space.
Here’s the Breakdown of Surface Pro Storage
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Total storage as reported by Windows (binary system)
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59*
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119*
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Storage space reserved for Windows recovery tools
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-9
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-9
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Available storage reported by File Explorer
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50
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110
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|
Windows 8 Pro and built-in apps
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-21
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-21
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Available storage reported by File Explorer
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29
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89
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The 64GB version has around 59GB actual space(as per the binary system)with around 21GB memory utilized by the Operating System and built in apps and around 9GB of space reserved for Windows Recovery Tools,giving users around 29GB to play with.Similarly the 128GB version which has around 119 GB of actual space(as per the binary system) with same amount of storage space accumulated by the Operating System and the Windows Recovery Tools as with the 64GB version ,giving the user 89GB to play with.
If you still feel that much amount of memory is not enough,you will always have the choice to put in a Micro SD Card to bump up the memory or use an external storage device.
Source[Surface]