According to MP-JWT 1.1 specification, 1024 and 2048 bit key sizes must be supported. Though when there is JWT signed by 1024 bit long key presented to the server, it is rejected and client receives "Unauthorized" (code 401) message.

See chapter 9.2. Supported Public Key Formats:

Support for RSA Public Keys of 1024 or 2048 bits in length is required. Other key sizes are allowed, but should be considered vendor-specific.