Try This
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select file_name, hwm, blocks total_blocks, blocks-hwm+1 shrinkage_possible
from dba_data_files a,
( select file_id, max(block_id+blocks) hwm
from dba_extents
group by file_id ) b
where a.file_id = b.file_id
/

Will tell u about the possible shrinkage in terms of blocks