When you drag your Offsite Stack onto your page, you will see the above stack appear on your page.
Set the Offsite URL in the stack setting to the webpage that you would like to be displayed. (The website you want is not showing up, please see the Offsite - Why Will My URL Not Work FAQ).
You can now setup your desired height and width settings for the iframe. The javascript tries its best to automatically determine the height, however, every browser interprets every site differently. Its recommended that you hard code a height to be used.