Hnnh. Everywhere I've seen the word "viewport" used, it has referred to the number of pixels on the screen, along with the aspect ratios of both the pixels, and the screen itself. I dabbled in OpenGL a long way back; this is probably where I was first introduced to the term.

I would call the concept which you are calling "viewport", "screen". e.g. "This phone has a much smaller screen than this nice desktop monitor."