From microsoft
Make sure you have enough free disk space. If you're currently running Windows 8, you need 3,000 MB of available space to install the 32-bit version of Windows 8.1, and 3,850 MB of available space to install the 64-bit version of Windows 8.1. On a Windows RT device, you need 2250 MB of available disk space to install Windows RT 8.1. For more info, see Tips to free up drive space on your PC