The above query gives the size all blocks whether they are filled with rows or not. Exporting and finding the size also may not be a perfect way as it gives only the difference in the dumpfile sizes. If you dont rquire it 100% exactly, then analyze the table to compute statistics and from dba/user_tables find out the avg_row_len, multiply this by the no. of rows.

The date field in Oracle requires 7 bytes.