414 means "URI is too large". Under Apache, this is a configurable limit, controlled by the LimitRequestLine configuration parameter. The maximum for this parameter appears to be 8190 by default, but even that can be hacked at compile time.

The W3 specs do not provide a "minimum maximum" or a "maxiumum maximum" for the length of a URI, so you really are on your own to see what will work in any given situation. There's no theoretical reason why a 50 megabyte GET would be disallowed, so the common chatter of "you can't have a URI longer than mumbleK bytes" has no basis in reality, because the right answer is "well, it depends".

-- Randal L. Schwartz, Perl hacker