hwwm

SELECT

    df.file_name,

    df.file_id,

    df.bytes / 1024 / 1024 AS current_size_mb,

    CEIL((NVL(MAX(e.block_id + e.blocks), 1) * ts.block_size) / 1024 / 1024) AS hwm_mb

FROM

    dba_data_files df

    JOIN dba_tablespaces ts ON df.tablespace_name = ts.tablespace_name

    LEFT JOIN dba_extents e ON df.file_id = e.file_id

WHERE

    df.tablespace_name = UPPER('YOUR_TS_NAME')

GROUP BY

    df.file_name, df.file_id, df.bytes, ts.block_size;