Material Design guidelines recommend making touch targets at least 48x48dp. Android’s device-independent pixels (dp) and Apple’s points (pt) are functionally equivalent. They refer to a baseline size that measures independent of the screen size — just like the CSS unit pixel (px).

Note that touch targets extend beyond the visual bounds of an element. For example, an icon may appear to be 24x24dp, but the padding surrounding it can comprise the full 48x48dp touch target. A 48x48dp touch target results in a physical size of about 9mm, regardless of screen size.