Steve,
There is actually an much easier way to find what you are looking for and make the change. The URL Length is something that is defined under File Types. So you go to Application Security > File Types > Allowed file types and you will see the column for URL Length. You will need to know what the file extension is on the end of the URL being blocked and adjust the appropriate one.
Nathan was correct though in saying that if you have learning turned on for Illegal URL length, you can go to Manual Policy Building > Traffic Learning and here you should see a violation for Length Errors, click on that and you should be able to find the block from there.
Mike