div size relative to screen size
===

I'm programming a html5 app with JQuery Mobile and Phonegap. I want to run them on many different devices. Because of that, I want to have everything relative to screen size. 

    <div id="pos" data-role="content"
			style="border: 5px solid silver; margin-left: auto; margin-right: auto; width: 250px; height: 250px;">

I use this div Element to display a google map inside with a little border around. 

How can I adjust the size for example 80% of the screen size?

I tried it with width: 90%; height: 90%, but the result is really bad. Its not relative to screen, it lokks like relative to the content.

And the size of the border is fixed with 5px, is it possible to insert here a nice argument to have it relative to screen size?


I hope someone can help!

Thanks!