Hmmmm..... I am not sure you can do that, because the URL length control is under to File Type not the specific URL, so you would be adjusting the entire .do file type category. The only way I could think of to do what you are asking, is to create another HTTP Class/Policy for just that URL. You can go and create an HTTP Class and then Check the box that says URI Paths, select Match Only in the drop down box and put in /AdminCategories.do. Then you can configure the URL length specically for that URL. Obviously this is a bit messy in that you now have a separate policy for just the one URL, but you keep an edge on the security. Honestly though I would probably just adjust the URL Length for .do in the main policy and call it a day, as long as your back end server/application is configured well and can handle other .do URLs of the same length there is really not much of a concern.
Mike