Hi amar,
Remove the width from the tinyMCE.init.
Set the width of the textbox to 100%.
Place a DIV around the TextBox and set the width and height of the DIV to whatever you want for a width and height.
HTML
<script type="text/javascript" src="//tinymce.cachefly.net/4.0/tinymce.min.js"></script>
<script type="text/javascript">
tinymce.init({
selector: 'textarea'
});
</script>
<div style="width: 500px; height: 200px;">
<textarea rows="2" cols="20" id="TextBox1" style="width: 100%;"></textarea>
</div>
<br />
<br />
<div style="width: 200px; height: 500px;">
<textarea rows="2" cols="20" id="Textarea2" style="width: 100%;"></textarea>
</div>
Demo