client_max_body_size per $request_uri

[skyroski]

For the record, I used a combination of a similar issue in the mailing
list and a combination of what @Valentin suggested.

In the end, this is what works for me:

if ($request_uri ~* upload$) {
  rewrite ^/(.*)$ /upload/$1 last;
}

location ~ /upload/(.*)$ {
  internal;
  client_max_body_size 200M;
  fastcgi_param PHP_VALUE ....;
  fastcgi_pass php;
  fastcgi_param SCRIPT_FILENAME $document_root/index.php;
  include fastcgi_params;
  fastcgi_param QUERY_STRING /$1;
  fastcgi_param DOCUMENT_URI /index.php;
  fastcgi_param SCRIPT_FILENAME /index.php;
}

All the rest I didn't have to change with the exception of adding an
extra clause to the location \.php$ block to restore PHP_VALUE to
default settings. 

I understand around-abouts why IF is evil but if it's supposed to be
safe for rewrites and return then I'd rather take advantage of that
instead of using location / with try files so that I can deal with query
strings attachments easier and it ends up shorter to implement.

@Valentin's solution would have meant I had to maintain 3 different
fastcgi_pass php location blocks as one would also be required for
location ~ upload$ {. 

Thanks for all the hints and tips.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,221727,221730#msg-221730