Finding out how much space a library uses

The library total:

SELECT SUM(OBJSIZE) / 1024 / 1024 AS MB
  FROM TABLE(QSYS2.OBJECT_STATISTICS('PRODLIB', '*ALL'))

But the total is of little use: what matters is who is taking the space.

SELECT OBJNAME, OBJTYPE,
       OBJSIZE / 1024 / 1024 AS MB,
       LAST_USED_TIMESTAMP
  FROM TABLE(QSYS2.OBJECT_STATISTICS('PRODLIB', '*ALL'))
 ORDER BY OBJSIZE DESC
 FETCH FIRST 20 ROWS ONLY

The largest libraries on the system, when you do not know where to start:

SELECT OBJNAME AS LIBRARY, OBJSIZE / 1024 / 1024 AS MB
  FROM TABLE(QSYS2.OBJECT_STATISTICS('QSYS', '*LIB'))
 ORDER BY OBJSIZE DESC
 FETCH FIRST 20 ROWS ONLY

Releases. QSYS2.OBJECT_STATISTICS is available from 7.2; on 7.1 it came with a PTF. The LAST_USED_TIMESTAMP column was added after the function itself: if the query answers SQL0206 — column not found — drop it and the rest still works.