Just a small hack, but worked good on my ISPConfig 3 setup. Add the following:

nano /etc/postfix/main.cf

mailbox_size_limit = 104857600

Save and restart postfix service.

According to official postfix documentation:
message_size_limit (default: 10240000) The maximal size in bytes of a message, including envelope information. Note: be careful when making changes. Excessively small values will result in the loss of non-delivery notifications, when a bounce message size exceeds the local or remote MTA’s message size limit.

Additionally, the default mailbox size of 50M may prevent mail from being delivered, especially after increasing the permitted message size. To increase maximum per user mailbox size, add mailbox_size_limit = <size in bytes> to main.cf.

Additionally, as Ian Sparkes commented, if you are using a virtual mailbox configuration, you might need to set virtual_mailbox_limit = <size_in_bytes>.

You may use the postconf tool to check the currently configured value:

postconf message_size_limit