[monolithed]

Как-то так

<script type="text/javascript">
window.onload = function(){
	document.getElementById("element").onkeyup = function(){
		var getText = this.value;
		var getRegs = getText.match(/^.*(\r\n|\n|$)/gim);
		var setText = false;
		for(var i = 0; i < getRegs.length; i++){
			getText = getRegs[i].replace(/\r|\n/g, "");
			setText += getText.length ? Math.ceil(getText.length / 50) : 1;
		}
		this.rows = setText;
	};
};
</script>
<textarea rows="10" cols="50" id="element" style="overflow: hidden; min-height: 200px"></textarea>