Print CSS - a full page for an element

As pointed out by tomorrow__, this can be achieved by applying

width: 100vw; height: 100vh;

to the element.

Use a page-break-after

http://www.w3schools.com/cssref/pr_print_pageba.asp

#important_thing { 
  width:100%; 
  height:100%;
  page-break-after:always 
}

You may have to combine it with a page-break-before:always