The size of the datafiles (no redo, no tempfiles) can be calculated as shown:
select sum(bytes) from dba_data_files
how many does your datafiles take place in operating system
select sum(bytes) from dba_segments
allocated data (but not fully used by data)
select sum(bytes) from dba_free_space
datafiles are larger than data stored in it - this is unused space in datafiles
