Minimum Space Requirement for Windows 2012
According to Microsoft, the minimum disk requirement for a successful install of Windows 2012 is 32 GB.
However, to cater to Windows Updates and other applications ( e.g. email, website ) which you may use. It is recommended that at least 50 GB of disk space be set aside for your cloud server.
Please refer to the following article to know more.
Knowledge Base: Editing Windows Updates in Windows Server 2012