Autosize插件官方：https://github.com/jackmoore/autosize
案例代码：
<!DOCTYPE html>
<html>
<head>
<meta charset='utf-8' />
<title>Simple Autosize for textareas</title>
<style>
textarea {
padding: 10px;
vertical-align: top;
width: 200px;
resize:none;
outline:none;
}
textarea:focus {
outline-style: solid;
outline-width: 2px;
}
</style>
</head>
<body>
<h3>max-height 300px</h3>
<textarea style='max-height: 300px'>
The coconut palm (also, cocoanut), Cocos nucifera, is a member of the
family Arecaceae (palm family). It is the only accepted species in the
genus Cocos.[2] The term coconut can refer to the entire coconut palm,
the seed, or the fruit, which, botanically, is a drupe, not a nut. The
spelling cocoanut is an archaic form of the word.[3] The term is derived
from 16th-century Portuguese and Spanish coco, meaning "head" or "skull",[4]
from the three small holes on the coconut shell that resemble human facial
features.
</textarea>
<h3>no max-height</h3>
<textarea>
The coconut palm (also, cocoanut), Cocos nucifera, is a member of the
family Arecaceae (palm family). It is the only accepted species in the
genus Cocos.[2] The term coconut can refer to the entire coconut palm,
the seed, or the fruit, which, botanically, is a drupe, not a nut. The
spelling cocoanut is an archaic form of the word.[3] The term is derived
from 16th-century Portuguese and Spanish coco, meaning "head" or "skull",[4]
from the three small holes on the coconut shell that resemble human facial
features.
</textarea>
</body>
<script src='/autosize.min.js'></script>
<script>
autosize(document.querySelectorAll('textarea'));
</script>
</html>
.Autosize下载：
最好去下载官方最新的版本。
注意，以上案例代码中，还用css3样式去掉textarea标签的右下角拖动元素修改尺寸大小属性(resize:none)和轮廓线(outline:none)。
