What is the maximum possible length of a query string?

Microsoft Internet Explorer (Browser)
Microsoft states that the maximum length of a URL in Internet Explorer is 2,083 characters, with no more than 2,048 characters in the path portion of the URL. Attempts to use URLs longer than this produced a clear error message in Internet Explorer.

Microsoft Edge (Browser)
The limit appears to be around 81578 characters. See URL Length limitation of Microsoft Edge

Chrome
It stops displaying the URL after 64k characters, but can serve more than 100k characters. No further testing was done beyond that.

Firefox (Browser)
After 65,536 characters, the location bar no longer displays the URL in Windows Firefox 1.5.x. However, longer URLs will work. No further testing was done after 100,000 characters.

Safari (Browser)
At least 80,000 characters will work. Testing was not tried beyond that.

Opera (Browser)
At least 190,000 characters will work. Stopped testing after 190,000 characters. Opera 9 for Windows continued to display a fully editable, copyable and pasteable URL in the location bar even at 190,000 characters.

Apache (Server)
Early attempts to measure the maximum URL length in web browsers bumped into a server URL length limit of approximately 4,000 characters, after which Apache produces a "413 Entity Too Large" error. The current up to date Apache build found in Red Hat Enterprise Linux 4 was used. The official Apache documentation only mentions an 8,192-byte limit on an individual field in a request.

Microsoft Internet Information Server (Server)
The default limit is 16,384 characters (yes, Microsoft's web server accepts longer URLs than Microsoft's web browser). This is configurable.

Perl HTTP::Daemon (Server)
Up to 8,000 bytes will work. Those constructing web application servers with Perl's HTTP::Daemon module will encounter a 16,384 byte limit on the combined size of all HTTP request headers. This does not include POST-method form data, file uploads, etc., but it does include the URL. In practice this resulted in a 413 error when a URL was significantly longer than 8,000 characters. This limitation can be easily removed. Look for all occurrences of 16x1024 in Daemon.pm and replace them with a larger value. Of course, this does increase your exposure to denial of service attacks.