Use something like:

error_page 413 /413.html;
location = /413.html {
root /path/to/your/site;
}

To change the error page. If you want to change nginx's default
message, but not the error page, i guess you'd have to edit the code.
If none of the above please calrify.

http://wiki.nginx.org/Main has many good references.

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