I need to know the maximum length of JSON Web Token (JWT).
There is no information about it in the specs. Are there are no limitations in length?
As you said, there is no maximum length defined in the RFC7519 (https://www.rfc-editor.org/rfc/rfc7519) or other RFCs related to JWS or JWE.
If you use the JSON Serialized format or JSON Flattened Serialized format, there is no limitation and there is no reason to define a limitation.
But if you use the JSON Compact Serialized format (most common format), you have to keep in mind that it should be as short as possible because it is mainly used in a web context. A 4kb JWT is something that you should avoid.
Take care to store only useful claims and header informations.