The queue.size parameter permits to specify the maximum queue size in number of messages. While not technically enforced, there is a lower limit on this parameter. Setting it to very low values (roughly below 100 messages) is not supported and can lead to unpredictable results. Also, future version my automatically adjust to a safe lower bound and/or decide to fail queue startup in those cases. So if you use very low values, do so at your own risk.
Lower Bound for Queue Sizes