unique pair of columns

[DaPi]

Or is it

SELECT label, sid, s_size
  FROM the_table
WHERE (sid, s_size) IN
(select sid, s_size
 from the_table
 group by sid, s_size
 having count(*) = 1);

or perhaps

select MAX(label), sid, s_size
 from the_table
 group by sid, s_size