This isn't a LISTSERV setting.  The problem is on the recipient's end.
The mail sent through your list was larger than his/her system was
configured to accept.

>The following mail got rejected because its attachment exceeded the site
>limit. It had the following error:
>
>message size of 4461539 bytes exceeds site limit of 4120576 bytes
>
>Does anyone know where the site limit field is configured?
>
>Thanks