Question

What is the maximum URL length you can pass to the Wininet function, HttpOpenRequest?

Answer 1

HttpOpenRequest does not have a maximum length but server software you are targeting will likely have a limit on your URL length.

Apache (Server)

My 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. I used the
current up to date Apache build found
in Red Hat Enterprise Linux 4. 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 16×1024 in Daemon.pm
and replace them with a larger value.
Of course, this does increase your
exposure to denial of service attacks.

(from Boutell.com)