Check how many inch the user drags on the screen

Before I use Input.mousePosition to determine how far did the user drag on the screen. But for some features it is not reasonable. For example, I need the user to drag for half of the screen on the iPhone to activate a feature, you won't ask the user to drag for half of the screen on iPad (suppose they have the same resolution, i.e. Input.mousePosition gives the same value). Or iPad mini and iPad mini retina has the same device dimension and they have totally different resolution. I cannot depend on the value of Input.mousePosition. So what can I use? Thanksf or advice.