Edit in JSFiddle

var newH = $(window).height() / 2;
var newW = $(window).width() / 2;

var newParams = {
    'height':'height: ' + newH + 'px !important;',
    'width':'width: ' + newW + 'px !important;'
};

$('#foo').css({'cssText':newParams.height + newParams.width});

#foo {
    background-color:#faa;
}

<div id='foo'>TEST</div>