To place an iframe into a WordPress page or post, you need a plugin such as: “Embed Iframe”.
This can be installed via WordPress or downloaded from http://blog.deskera.com/wordpress-plugin-embed-iframe
You then use the code in the following format, on any page or post, where you wish the iframe to appear:
The two numbers being the width followed by the height.
The above example is stripped down from this one, with the height reduced from 4,000 to just 1,000.
to enable it to work and appear correctly.
I did try it with the 90% command in place, but I am sure that the iframe code stripped the % sign, and made the frame only 90 wide.
I therefore had to work out the actual width of the frame required.
83.1 - 605,760