[AndrewGSW]

For 100% height to work you need to give the parent containers this height as well: CSS

html {
    height: 100%;
}
body {
    height: 100%;
    overflow: auto; /* to prevent the body from appearing below the iframes */
}

But the height also includes the menu, etc., so the iframes will probably stretch beyond the browser viewport.

You could consider just giving the iframes a specific height and allowing scrollbars to appear.

Edited: You may need to set the iframe heights to 100% using CSS or the style attribute.