How to determine Oracle LOB storage footprint?
You can get the size in bytes of the BLOB column via DBMS_LOB.GETLENGTH(...).

To get a total size in bytes of column for the entire table SUM it up like this:

SELECT SUM(DBMS_LOB.GETLENGTH(my_blob_column)) as total_bytes
FROM my_table


If you've marked the BLOB column as deduplicated then you can run something like the following to get stats on the actual size in bytes used. Compare the used_bytes value before/after your enable settings that could change the size of the BLOB column (eg. dedupe, compression, etc).

declare  
    p_schema_name           varchar(256) DEFAULT 'THE_SCHEMA_NAME';
    p_table_name            varchar(256) DEFAULT 'THE_TABLE_NAME';
    --
    l_segment_name          varchar2(30);
    l_segment_size_blocks   number; 
    l_segment_size_bytes    number; 
    l_used_blocks           number;  
    l_used_bytes            number;  
    l_expired_blocks        number;  
    l_expired_bytes         number;  
    l_unexpired_blocks      number;  
    l_unexpired_bytes       number;  

begin
    FOR rec IN (select segment_name 
                from dba_lobs 
                where owner = p_schema_name
                  and table_name = p_table_name
                order by 1)
    LOOP
        dbms_output.put_line('Segment Name=' || rec.segment_name);

        dbms_space.space_usage( 
            segment_owner           =&gt; p_schema_name,  
            segment_name            =&gt; rec.segment_name, 
            segment_type            =&gt; 'LOB', 
            partition_name          =&gt; NULL, 
            segment_size_blocks     =&gt; l_segment_size_blocks, 
            segment_size_bytes      =&gt; l_segment_size_bytes, 
            used_blocks             =&gt; l_used_blocks, 
            used_bytes              =&gt; l_used_bytes, 
            expired_blocks          =&gt; l_expired_blocks, 
            expired_bytes           =&gt; l_expired_bytes, 
            unexpired_blocks        =&gt; l_unexpired_blocks, 
            unexpired_bytes         =&gt; l_unexpired_bytes 
        );   


        dbms_output.put_line('  segment_size_blocks       =&gt; '||  l_segment_size_blocks);
        dbms_output.put_line('  segment_size_bytes        =&gt; '||  l_segment_size_bytes);
        dbms_output.put_line('  used_blocks               =&gt; '||  l_used_blocks);
        dbms_output.put_line('  used_bytes                =&gt; '||  l_used_bytes);
        dbms_output.put_line('  expired_blocks            =&gt; '||  l_expired_blocks);
        dbms_output.put_line('  expired_bytes             =&gt; '||  l_expired_bytes);
        dbms_output.put_line('  unexpired_blocks          =&gt; '||  l_unexpired_blocks);
        dbms_output.put_line('  unexpired_bytes           =&gt; '||  l_unexpired_bytes);
    END LOOP;
end;
/


This code is a slightly modified version of the code in Oracle's post explaining how deduplication works. The original is available here: http://www.oracle.com/technetwork/articles/sql/11g-securefiles-084075.html.