Open `data/conf/postfix/extra.cf` and set the `message_size_limit` accordingly in bytes. See `main.cf` for the default value.

Restart Postfix:
=== "docker compose"

    ``` bash
    docker compose restart postfix-mailcow
    ```

=== "docker-compose"

    ``` bash
    docker-compose restart postfix-mailcow
    ```