Database manager asks you as a DBA about all the database size in the supported environment .
On 13.1 OMS repository:
SQL> select host_name,target_name, sum( FILE_SIZE/1024/1024/1024)
from mgmt$db_datafiles
group by host_name,target_name
order by host_name, target_name;
HOST_NAME TARGET_NAME SUM(FILE_SIZE/1024/1024/1024)
---------------------------------------- ---------------------------------------- -----------------------------
HOST1 DB1 38.65
HOST2 DB2 385.88
HOST3 DB3 27.52
...
..
.
OR we can use another another view mgmt$db_tablespaces :
SQL> select host_name,target_name,sum( TABLESPACE_SIZE/1024/1024/1024)
from mgmt$db_tablespaces
group by host_name,target_name
order by 1,2;