What\'s the maximum length of an HTTP GET request?
Is there a response error defined that the server can/should return if it receives a GET request that exceeds this
You are asking two separate questions here:
What's the maximum length of an HTTP GET request?
As already mentioned, HTTP itself doesn't impose any hard-coded limit on request length; but browsers have limits ranging on the 2 KB - 8 KB (255 bytes if we count very old browsers).
Is there a response error defined that the server can/should return if it receives a GET request exceeds this length?
That's the one nobody has answered.
HTTP 1.1 defines status code 414 Request-URI Too Long
for the cases where a server-defined limit is reached. You can see further details on RFC 2616.
For the case of client-defined limits, there isn't any sense on the server returning something, because the server won't receive the request at all.