'Retrieve the size of a message, or, if none is specified, the size of
each message in the mailbox.
@type i: L{int} or L{None}
@param i: The 0-based index of the message.
@rtype: L{int}, sequence of L{int}, or L{Deferred <defer.Deferred>}
@return: The number of octets in the specified message, or, if an
index is not specified, a sequence of the number of octets for
all messages in the mailbox or a deferred which fires with
one of those. Any value which corresponds to a deleted message
is set to 0.
@raise ValueError or IndexError: When the index does not correspond to
a message in the mailbox.  The use of ValueError is preferred.'

def listMessages(self, i=None):

    return []
