You can set the Maximum query string length by adding this setting in your web.config file under <system.web> section:
<httpruntime maxrequestlength="16384" maxquerystringlength="99999999" />
But generally you should not have to do this. Try keeping your query strings within the default limits.
Update: Despite this setting the maximum length would be dependent on the browser you use, like aspnet_regiis has mentioned.