Maximum URL characters length
Microsoft Internet Explorer has a maximum uniform resource locator (URL) length of 2,083 characters. Internet Explorer also has a maximum path length of 2,048 characters. This limit applies to both POST request and GET request URLs.
If you are using the GET method, you are limited to a maximum of 2,048 characters, minus the number of characters in the actual path.
However, the POST method is not limited by the size of the URL for submitting name/value pairs. These pairs are transferred in the http://www.pr-inside.com/ header and not in the URL.
Google does not like session id's in url's. Rule of thumb is not to make querystrings too long, but I doubt anybody knows for sure how long.