How can I make the drawing area 100% of the viewport?

I would like to modify the code so that it acts as sort of a background on my webpages. Someone reading an article can then underline or highlight any word or draw a little picture anywhere they want, even if the article is super long.

Can anyone tell me how to modify Ghostwriter's code so that the drawing area is 100%, rather than 500 or 750 pixels?

Thanks.
