Javascript - 문서 스크롤 높이(문서 세로 길이)

var body = document.body,

    html = document.documentElement;

var height = Math.max( body.scrollHeight, body.offsetHeight, 

                       html.clientHeight, html.scrollHeight, html.offsetHeight );

출처 : http://stackoverflow.com/questions/1145850/how-to-get-height-of-entire-document-with-javascript