How long can the _id string for CouchDB documents be?

What is the maximum length for the _id field of CouchDB documents? Where are 开发者_开发技巧the specifications for this?

---

From this mail on the users mailing list:

The max size for a document is 4GB, any property (including the _id) is included in this.

Also, here's another tidbit about IDs and their affect on performance: http://wiki.apache.org/couchdb/Performance#A_id