The radiusX read-only property of the Touch interface returns the Y radius of the ellipse that most closely circumscribes the area of contact with the touch surface. The value is in CSS pixels of the same scale as Touch.screenX.
This value, in combination with Touch.radiusX and Touch.rotationAngle constructs an ellipse that approximates the size and shape of the area of contact between the user and the screen. This may be a large ellipse representing the contact between a fingertip and the screen or a small one representing the tip of a stylus, for example.