89c0777d_HiveMQ_1_4_1___User_Guide_Table_3__Static_properties_Description
[Property Name] global.maxClientIdLength [Default Value] 23 [Description] 
The maximum length of client ID. The MQTT v3.1 spec defines a limit of 23. If this is not enough for your use case, set it to a higher limit
HiveMQ 1.4.1 - User Guide
Description