Ok everyone who works on touch based devices, this is an important exercise.
Take your screen resolution and DPI, and find the size of your fingerprint in pixels for your device. For example, my fingertip covers about 40x50 pixels on my Pixel2XL, if I barely touch.
This is the *minimum* size you are allowed to make a widget.
Yes, keyboards are all doing it wrong. Yes, *you* are probably doing it wrong (and so have I).
Please start doing it right!