var RSZ = {};
RSZ.h_main = $('main').clientHeight;
RSZ.h_sub = $('sub').clientHeight;
RSZ.h_header = $('header').clientHeight;
RSZ.h_footer = $('footer').clientHeight;

window.onload = resize;
window.onresize = resize;

function resize() {
    var h_body = document.body.clientHeight;
    var h_content = Math.max(RSZ.h_main, RSZ.h_sub, h_body - RSZ.h_header - RSZ.h_footer)
    $('sub').style.height = $('main').style.height = h_content + 'px';
}