CALIBRATION IN PROGRESS... In order to calibrate your screen, you'll need either a phone, or a credit card/driver's license (just for size reference). Once you choose your preferred way of calibration, you'll have to adjust its onscreen size in the next step.
You will be able to compare the relative size of phones, but they will not be displayed on your screen as big as in real life.
CALIBRATION IN PROGRESS... Adjust the size of the phone/card using the Plus/Minus controls on the right, until it is displayed on the screen as big as it is in real life.