logstash's monitoring API only reports the bytes used in the DLQ.
If it were to report the configured bytes for the DLQ like what it does with the persistent queue, then my monitoring tool can figure out what percentage of the DLQ is being consumed and I can avoid telling my monitoring tool what the current configured size of my DLQ is. The monitoring tool would gather that from the logstash API.
Please opine and/or create a bug at the logstash github repo.