What is the size limit of a post request?

It depends on a server configuration. If you're working with PHP under Linux or similar, you can control it using .htaccess configuration file, like so:

#set max post size
php_value post_max_size 20M

And, yes, I can personally attest to the fact that this works :)

If you're using IIS, I don't have any idea how you'd set this particular value.

You may want to use a different mechanism anyway. Referrers are easily spoofed. Checking referrers really isn't a good security solution, and if they're going to cause you headaches like this, maybe you want to find another way.

For example, the server generating the first page could add an authorization token to the URLs to the second server, and the second server could check that the tokens are valid. This way, all of the details are under your control, and the only browser behavior you're counting on is that the full URL is sent to the second server.

The username:password@example.com construct was removed as a security feature. Storing passwords in plain text is a bad idea to start with, and this form of the url was a frequently used attack vector for phishing attacks. The article you link to lists some work-arounds... can you not just prompt the user to enter a password?

If you give a little more details about what you're doing, it's possible a better solution may be found.