How to get the parent iframe size in the embedded page js code?

How to get the parent iframe size in the embedded page js code?

I am writing a signage html page to be shown in full screen automatically.
I use screen.width and .height to get the client's screen size. Now for
the edit page which users can add/delete contents on the signage, I would
like to let users preview the edited result without going to signage site
in fullscreen. So I plan to have a iframe has the same ratio as the client
screen and embedded the signage page into the iframe on the edit page.
The thing is how I can let the signage page change the size to fit in the
iframe? Or how I can get the parent page's iframe from the embedded page?
The screen.width and $(document).width seem to have the same result.