According to the LENGTH options in CICS commands
When a CICS command offers the LENGTH option, it is generally expressed as a signed halfword binary value. This puts a theoretical upper limit of 32 763 bytes on LENGTH. In practice (depending on issues of recoverability, function shipping, and so on), the achievable upper limit varies from command to command, but is somewhat less than this theoretical maximum. To be safe, do not let the value assigned to the length option for any CICS command exceed 24KB.