The size is dicated by what you want to put on it.
FMS uses temp files that can grow as large as the real files.
Backups depends on how many backups sets you configure to keep
Progressive backups need a least 3 times the size of your files
Given all that, add ample room for growth and keep monitoring.
On the root drive, allow for expansion of Windows updates etc.
Given that disk size is relatively cheap: don't skimp, go big.