Have you considered the <textarea> element?
Or, if you want to use a normal textbox, you could try setting the CSS style, rather then use the ancient "height" attribute.
Like:
- <input type="text" style="height: 100px;" />
The best way would of course be to include this in an external CSS stylesheet, but that is hardly worth creating one if you don't already use them.
Note that this will not add more lines to the box, it will only stretch the height around that single line. If you need multiple lines, use <textarea>