I have tinymce editor(textarea) and one div. Whenever I type inside the text editor, it shows at the preview div which is (200px) in real time which is looks alike stackover
I may be misunderstanding your issue, but it seems like all you need is a bit of CSS, specifically a max-width (for all the non-idiotic browsers) and a width with a wacky IE expression (for IE).
For instance
max-width:200px;
_width:expression(document.body.clientWidth > 200? "200px": "auto" );
When you combine that with the CSS you already have, it seems like it should work.