To check the current settings for postifx:

postconf mailbox_size_limit
The default mailbox size is 51200000.

postconf message_size_limit
The default message size is 10240000.

To change it:

vi /etc/postfix/main.cf
…
mailbox_size_limit = newmailboxsize
message_size_limit = newmessagesize
…

Restart postfix