If you run this command,
sudo dpkg-reconfigure postfix
you'll eventually get to a screen like this:
┌──────────────────────────────────────────────┤ Postfix Configuration ├───────────────────────────────────────────────┐
│ Please specify the limit that Postfix should place on mailbox files to prevent runaway software errors. A value of │
│ zero (0) means no limit. The upstream default is 51200000. │
│ │
│ Mailbox size limit (bytes): │
│ │
│ 0___________________________________________________________________________________________________________________ │
│ │
│ │
│ │
└──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
Right now, postfix is complaining that the maximum size of a single email is larger than the maximum size of the entire mailbox. You could simply type in a lager value for the entire mailbox at this screen, and it should fix that problem. 0 means no limit on the maximum size. I think the default maximum email size is 10240000, so if you do set a mailbox size limit, make sure it's more than that.