1  select table_name, column_name, data_length from dba_tab_columns
  2  where column_name = 'DB_NAME'
  3   and data_type = 'VARCHAR2'
  4* order by 1,2
SQL> /

TABLE_NAME                     COLUMN_NAME                    DATA_LENGTH
------------------------------ ------------------------------ -----------
DBA_HIST_DATABASE_INSTANCE     DB_NAME                                  9
GV_$ASM_CLIENT                 DB_NAME                                  8
GV_$LOGMNR_DICTIONARY          DB_NAME                                  9
GV_$LOGMNR_LOGFILE             DB_NAME                                  8
GV_$LOGMNR_LOGS                DB_NAME                                  8
GV_$LOGMNR_SESSION             DB_NAME                                128
LOGMNRG_DICTIONARY$            DB_NAME                                  9
LOGMNRT_DICTIONARY$            DB_NAME                                  9
LOGMNR_DICTIONARY$             DB_NAME                                  9
MGMT$RACDB_INTERCONNECTS       DB_NAME                               1024
V_$ASM_CLIENT                  DB_NAME                                  8
V_$LOGMNR_DICTIONARY           DB_NAME                                  9
V_$LOGMNR_LOGFILE              DB_NAME                                  8
V_$LOGMNR_LOGS                 DB_NAME                                  8
V_$LOGMNR_SESSION              DB_NAME                                128
WRM$_DATABASE_INSTANCE         DB_NAME                                  9