Make Div 100% of grandparent container?

Question

You can use the viewport’s height and width.

For example, the following class will make the element the size of the viewport.

.fullscreen {
    width: 100vw;
    height: 100vh;
}

jsFiddle Demo

This is a pure CSS3 solution, but it has some browser support issues.

If you just want to strech an element on the entire screen you can use a different approach.

jsFiddle Demo

.fullscreen {
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
}