Pages normally don't have a fixed height.

If you set a page or some element on a page to a fixed height using CSS, then you can size things in terms of %.

You'll need to use a bit of Javascript to set the page height.

Here's one way to get the device height:

var viewportHeight = document.documentElement.clientHeight;

Here's another (that I haven't tried, but should work) because jQuery Mobile sets the device height as min-height CSS for the page. (And assuming you already have a $page variable with the page.)

var viewportHeight = parseFloat($page.css('min-height'));

Then, you can:

$page.height(viewportHeight + 'px');