Edit in JSFiddle

$("#setwidth").click(function () {
    $("body").css("width", "50%");
});
$("#setheight").click(function () {
    $("body").css("height", "4em");
});

<button id="setwidth">设置body宽度</button>
<button id="setheight">设置body高度</button>

<h1>Lorem ipsum</h1>
<p>Lorem ipsum</p>
<p>Lorem ipsum</p>

body {
    border: 1px solid;
    background: lightblue;
}