Maximum length of HTTP GET request
The limit is dependent on both the server and the client used (and if applicable, also the proxy the server or the client is using).
Most web servers have a limit of 8192 bytes (8 KB), which is usually configurable somewhere in the server configuration. As to the client side matter, the HTTP 1.1 specification even warns about this. Here's an extract of chapter 3.2.1:
Note: Servers ought to be cautious about depending on URI lengths above 255 bytes, because some older client or proxy implementations might not properly support these lengths.
The limit in Internet Explorer and Safari is about 2 KB, in Opera about 4 KB and in Firefox about 8 KB. We may thus assume that 8 KB is the maximum possible length and that 2 KB is a more affordable length to rely on at the server side and that 255 bytes is the safest length to assume that the entire URL will come in.
If the limit is exceeded in either the browser or the server, most will just truncate the characters outside the limit without any warning. Some servers however may send an HTTP 414 error.
If you need to send large data, then better use POST instead of GET. Its limit is much higher, but more dependent on the server used than the client. Usually up to around 2 GB is allowed by the average web server.
This is also configurable somewhere in the server settings. The average server will display a server-specific error/exception when the POST limit is exceeded, usually as an HTTP 500 error.
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.