Cisco Unity Express (CUE) has a number of restrictions on the mailboxes, as well as send and receive limits that may not be obvious.
The storage space allowed is determined by the license installed on the system.
The default mailbox size is determined by this equation:
(#minutes-storage) / (#total-mailboxes)
For example, 100 personal mailboxes, 20 general delivery mailboxes, 6000 minutes storage (100 hours). Default mailbox size = (6000)/(100+20) = 353 minutes.