Try this!!
<iframe id="Iframe1" src="~/Aspx/Popup/PanelMembers.aspx" onload="setSize()"runat="server"></iframe>
function setSize() {
var iframeElement = parent.document.getElementById('Iframe1');
var height = document.documentElement.clientHeight - 100;
iframeElement.style.height = height + "px";
}