When exceeding the max request length you get a HTTP 500 error code.
If you check the Application Event viewer on the server side you might see errors such as this:
Event code: 3004
Event message: Post size exceeded allowed limits.
Event time: 10/24/2008 5:11:39 PM
Event time (UTC): 10/24/2008 10:11:39 PM
Event ID: 640ae5c3a03349b18112b5289fd09989
Event sequence: 12
Event occurrence: 3
Event detail code: 0
To solve this problem override your web.config with the following configuration element.
Also update the executionTimeout to allow more time before it shuts down the request.
<httpRuntime maxRequestLength="11000" executionTimeout="60" />
<!-- 50MB max request size-->
More information can be found at the MSDN site