Try it Editor

<!Doctype html>
<html>
<head>
    <title>Responsive Textarea</title>
    <style>
        .container {
            max-width: 820px;
            margin: 0px auto;
            margin-top: 50px;
        }
        .comment {
            float: left;
            width: 100%;
            height: auto;
        }
        .commenter {
            float: left;
        }
        .commenter img {
            width: 35px;
            height: 35px;
        }
        .comment-text-area {
            float: left;
            width: 100%;
            height: auto;
        }

        .textinput {
            float: left;
            width: 100%;
            min-height: 75px;
            outline: none;
            resize: none;
            border: 1px solid grey;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="comment">
            <textarea class="textinput" placeholder="Comment"></textarea>
        </div>
    </div>
</body>
</html>