As seen here https://www.rfc-editor.org/rfc/rfc1929 , I would assume that it is either 255, or unlimited in length.
What is the longest possible username and/or password you can use for authenticating a SOCKS 5 proxy?
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I read the same document, and read it as specifically stating that username and password are 1-255 octets long. Assuming single byte encoding this would be 255 chars max.