Here's an example, although you have to click twice to get the right position, which may possibly be a somewhat related bug that I'll post a report for. dropbox.com/s/dkmkdrbqaot4g60/canvassize.c3p
The black bars (letterbox scale), zoomed in (scale outer), or zoomed out (scale inner), depends on your fullscreen scaling mode. These are out of your control, because you cannot control the aspect ratio of the user's window (browser size) or phone screen. You'll just have to design accordingly, to see which mode is most suitable for you.