I looked at the jplayer API , and it looks like you can set the size of jplayer full screen mode even inside the iframe page !
Their document explains how to use Set Full Size on an iframe page.
After that, the Set full size link shows the parameters, of which 2 of 3 the default values that interest you are shown here:
width String : (Default: [Video:"100%"] [Audio:"0px"]) : The width as a CSS rule. height String : (Default: [Video:"100%"] [Audio:"0px"]) : The height as a CSS rule.
To view a live example of setting this parameter is easy thanks to the jPlayer development test on this page.
Here are the test instructions:
1. Click the blue link : "html, flash" at the top of the page.
2. Scroll down to the first element of the line called setMedia(A:{m4v}) and click on it.
3. Finally, set the full-screen size by sizeFull to a gray selection called sizeFull and set the value to 360p .
Now click play on the created jplayer below! Then, during playback, pressing the full-screen button increases jplayer to its maximum size using 360p .
For iframe page requirements, maximum size will not be the default settings, as this is a 100% iframe container.
Instead, use JavaScript to specify the width and height of the client screen, thereby making it fullscreen outside the iframe page. Example:
screen.height; screen.width;
Of course, the Width and height of the video can be a percentage, as shown or in pixels. Next, the string is wrapped in quotation marks as strings, as shown in the above example.
[Video: "'" + screen.height + "px" + "'"]
[Video: "'" + screen.width + "px" + "'"]
The above may not work if the iframe is not in the same domain, but I see that this is not a problem for you.