I have already looked at your install. There is no CSS to ensure a max-width: 100% is set on the iframe. The reason it looks correctly in your blog is because your template specifically has CSS to style iframes with a container div that have a CSS class of wf-iframe-container, which you didn't add to your textarea editor field usage. Add the below to your Joomla template CSS file or to CB template CSS override usage and it'll prevent iframes from overflowing.
Code:
iframe {
max-width: 100%;
}