Obter tamanho das tabelas no Firebird
/* Display table names and record counts */
SELECT RDB$RELATIONS.RDB$RELATION_NAME,
CASE
WHEN RDB$INDICES.RDB$STATISTICS = 0 THEN 0
ELSE CAST(1 / RDB$INDICES.RDB$STATISTICS AS INTEGER)
END
FROM RDB$RELATIONS
LEFT JOIN RDB$RELATION_CONSTRAINTS
ON RDB$RELATIONS.RDB$RELATION_NAME = RDB$RELATION_CONSTRAINTS.RDB$RELATION_NAME
AND RDB$CONSTRAINT_TYPE = 'PRIMARY KEY'
LEFT JOIN RDB$INDICES
ON RDB$RELATION_CONSTRAINTS.RDB$INDEX_NAME = RDB$INDICES.RDB$INDEX_NAME
WHERE RDB$VIEW_BLR IS NULL AND RDB$RELATION_ID >= 128
ORDER BY 1;
fonte: Demoncyber's Weblog