Don't. Find a different solution.
The problem is that the max length of a query string is not large, but it also isn't defined. IE has a maximum length of the entire URL + Query string of 2,083 characters http://support.microsoft.com/kb/208427
] and that is characters, not bytes - since the URL and querystring are supposed to be human readable, you are looking at a lot less actual bytes than that!
The fact that you have to ask the question indicates that you have made a very wrong decision somewhere, and you need to go back and re-think your design a little. I can't suggest a "better way" because I have no idea why you think this is necessary in the first place!