The following works for me:-
DokuWiki Detritus
snippet from conf/userstyle.css
.big_iframe{
border:none;
margin:0;
position:fixed;
top:0px;
left:0px;
bottom:0px;
right:0px;
width:100%;
height:100%;
padding:0;
overflow:hidden;
z-index:999999
}
[/color]
data/pages/playground/iframe.txt
====== Iframe ======
<html>
<iframe class="big_iframe" src="https://www.dokuwiki.org/">
<p>Your browser does not support iframes. (which may not be a bad thing)</p>
</iframe>
</html>
[/color]
By "works" I mean it fills the entire viewport with the iframe. And no, you can't access DokuWiki links as they are in a layer beneath the iframe.
screenscrape here
Try the style and html I've posted above, then let me know what's not working/doesn't suit your usecase. It might be helpful if you could give me link to your iframe page.