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.

The url portion of a request (GET and POST) can be limited by both the browser and the server - generally the safe size is 2KB as there are almost no browsers or servers that use a smaller limit.

The body of a request (POST) is normally* limited by the server on a byte size basis in order to prevent a type of DoS attack (note that this means character escaping can increase the byte size of the body). The most common server setting is 10MB, though all popular servers allow this to be increased or decreased via a setting file or panel.

*Some exceptions exist with older cell phone or other small device browsers - in those cases it is more a function of heap space reserved for this purpose on the device then anything else.

Also, in PHP.INI file there is a setting:

max_input_vars

which in my version of PHP: 5.4.16 defaults to 1000.

From the manual: "How many input variables may be accepted (limit is applied to $_GET, $_POST and $_COOKIE superglobal separately)"

Ref.: http://www.php.net/manual/en/info.configuration.php#ini.max-input-vars