Pixels are how computers, tablets and phones work, but you can't measure your fingers or eyes in pixels.
Pixels aren't even one thing; device pixel scales vary from one device to the next, so we need to design in real world units, and the easiest way to do that is just to toss your design on a screen, and measure everything. Not with some fancy screen mirroring software, but with this simple plastic measuring tool.
This is actually the third version of the Touch Template. I have done a lot (I mean a lot!) of research into how people really hold and touch their phones, tablets and phablets. The Touch Template doesn't just have little rulers and type scales, but suggests sizes you need to make your words, and gives guidelines for the size of touch targets. That's the circles on the right side, since touch accuracy varies by part of the screen.
This is already used by over a thousand designers, developers and testers in two dozen countries. People regularly tell me how useful it is, especially being universal and pocket sized so they can pull it out of their wallet at any time. I hope you agree.
4ourth Mobile Touch Template