尝试一下:

<div id="frame"></div>

#frame {
    position: absolute;
    left: 50%;
    top: 50%;
    margin: -100px 0 0 -100px;
    width: 200px;
    height: 200px;
    background-color: #999;
}

$(function() {
    var w = 400, h = 400;
    $("#frame").animate({
        width: w,
        height: w,
        marginLeft: -w/2,
        marginTop: -h/2
    }, 1000);
});

http://jsfiddle.net/GVj83/