First, there is the operating system and application programs. Linux is fairly space efficient, and normally does not require any more than 10GB of storage for the operating system and most server-oriented programs.
Next, you need to think about storage that is specific to the server. This requirement depends on what you plan to do with the server.
Don't forget to allocate some disk space for ``swap space''. A ``swap space'' is a portion of a hard disk that is used as slow system memory. The rule of thumb is that the swap space should have about the same size as the amount of memory.