It will also generate error when you pass large string in ajax call parameter.
so for that alway use type post in ajax will resolve your issue 100% and no need to set the length in web.config.
// var UserId= array of 1000 userids
$.ajax({ global: false, url: SitePath + "/User/getAussizzMembersData", "data": { UserIds: UserId}, "type": "POST", "dataType": "JSON" }}