How can I get the actual display size in canvas?

I am making an element in Animate CC (html5 canvas) that will be integrated in an html5 page with WordPress.

This element contains a navigation bar. Apparently this navigation bar looks to small when displayed on mobile phones.

I can tell it to be bigger depending on the width of the window, but that's in pixels and a "normal" display in desktop browsers uses less pixels than a very small display on mobile phones. They use different DPI.

I tried to use "Capabilities.screenDPI" but Html5 Canvas doesn't seem to recognize this command.

Bottom line - How can I get the actual display size, in inches or millimeters?

Note: I am an animator, not a programmer. I'm doing this in AS3 and learning it as I go but advanced JS is beyond my skill set...