How much space does an Oracle table take up ?

The following queries can be used to find out how much space a table takes up. The first query gets the block size used in this database. The second one shows how many blocks are used for a table.

select value
  from v$parameter
 where name = 'db_block_size';

select blocks, last_analyzed
  from all_tables
 where owner = 'X'
   and table_name = 'Y';

The blocks field will have a valid value if the table has been analyzed recently, that is why the last_analyzed field is selected as well.