Returns the size of a mailbox.
mailbox-size mailbox => size
An non-negative integer.
mailbox-size returns the size of mailbox, which is the number of objects that can be added without it growing.
mailbox-send automatically expands the mailbox indefinitely when it is full, but mailbox-send-limited and process-send will expand it up to some specified limit.
LispWorks User Guide and Reference Manual - 20 Sep 2017