select blocks, empty_blocks, num_freelist_blocks
where owner = 'MYSCHEMA'
and table_name = 'MYTABLE';
The total number of blocks allocated to the table is blocks + empty_blocks + num_freelist_blocks.
blocks is the number of blocks that actually contain data.
Multiply the number of blocks by the block size in use (usually 8KB) to get the space consumed - e.g. 17 blocks x 8KB = 136KB.