Service Unavailable - Zero size object - meaning
The server is temporarily unable to service your request. Please try again later.
Steam is throwing up an error message, here is what it means.
This is a php memory error. Steam will need to do this to correct (not your problem) but interesting.
There are more than one directive you need be concerned with when learning how to configure php.ini to upload big files [webmasterworld.com].
And don't forget that your HTTP server may come into play as well. For example, on Apache there is the limitrequestbody [httpd.apache.org] configuration directive.