for a QueryString, the actual limit depends on the browser being used, as well as the server software. Generally speaking, you are safe assuming that anything below 1800 characters of data will work, although some of the older, off-label browsers have a limit of 1500 characters. If you use the POST method to transmit data, the limits are 2MB for IIS 4.0 and 128KB for IIS 5.0.
The limit for an individual parameter in the QueryString is 1024 characters.