Set the rows attribute of the textarea element to "3".

<html>
<body>
<img id="profilePic" src="https://mimo.app/r/profile.png" height="45">
<textarea id="tweetText" rows="3" cols="18" placeholder="What's happening?"></textarea>
<br>
<button>Tweet</button>
</body>
</html>]]>

Make sure to code rows="3".