When I try to upload a file to my site, I\'m getting the Nginx \"413 Request Entity Too Large\" error, however in my nginx.conf file I\'ve already explicitly stated the max size
Not knowing the version of your nginx build and what modules it was built with makes this tough, but try the following:
Copy your client_max_body_size 300M; line into the location / { } part of your vhost config. I'm not sure if it's overriding the default (which is 1 MB) properly.
Are you using nginx_upload_module? If so make sure you have the upload_max_file_size 300MB; line in your config as well.