To create a full window iframe:

<iframe src="http://192.168.0.1/login.php" style="border: 0; position:absolute; top:0; left:0; right:0; bottom:0; width:100%; height:100%">

HTML email

https://litmus.com/blog/html-email-coding-101-infographic

http://visual.ly/code-therapy-16-tips-troubleshooting-your-html-email

For media queries based on width to work on mobiles, you need to add the following meta tag to the html file:
<meta name="viewport" content="width=device-width">