DATABASE NAME MAX SIZE in ORACLE

This is for 11gR2:

The length of DB_NAME is limited to 8 characters;

DB_UNIQUE_NAME is limited to 30 characters. DB_UNIQUE_NAME  can contain alphanumeric, underscore (_), dollar ($), and pound (#) characters, but must begin with an alphabetic character.

INSTANCE_NAME defaults to ORACLE_SID. Oracle document does not specify the limit for INSTANCE_NAME length. However if you describe V$INSTANCE view you can see that column INSTANCE_NAME  has VARCHAR2(16), indicating the  limit of 16 characters.

Because INSTANCE_NAME defaults to ORACLE_SID you need to be careful when deciding SID before creating a database. Oracle does not clearly define the limit for SID (system identifier). Following are what I found from two 11gR2 documents about ORACLE_SID.