<iframe src="https://somesite.com/somepage.aspx" onload="this.width=screen.width - ((screen.width <= 768) ? 100 : 500); this.height=screen.height - 100">
<iframe src="https://somesite.com/somepage.aspx" onload="this.width=screen.width - ((screen.width <= 768) ? 100 : 500); this.height=screen.height - 100">