$('div.square').animate({width:'150px',height:'150px'},'slow');
※CSSプロパティで以下の様に設定
div.square{ pcition:relative, top:100px; width: 100px; height: 100px; margin:3px; padding:3px; float:left border:1px solid #000; } div#left{background-color:#cf9;} div#right{background-color:#fc9;}
<div id="left" class="square" style="float:left"></div> <div id="right" class="square" style="float:left"></div>