It bytes column is available in user_extents view.

select sum(bytes) from user_extents group by 1

Hi

select owner segment_name bytes/1024/1024 Size in Mb from dba_segments where segment_type <TABLE_NAME>

This is total size physicall occupied

for actual size:

Analyze the table first using DBMS_STATS package and run the follwoing command.

select table_name num_rows* avg_row_len from dba_tables where table_name <TABLE_NAME>

for accurate you can include pctfree parameter also.

Padmanathan