Edit->Edit innerHTML of the container box.
The style attribute accepts any CSS. So width, height, background, etc …
Note: I set a maxlength of 2 (this makes the input only except 2 characters) You can take this out if you need.
Sure you can change the tag to a textarea example:
<textarea cols="25" rows="7"></textarea>
look into localStorage. There is a post here that talks about using it specifically in an iBook widget.