1. The outer part of the new home button is exactly the same size as the Phone app and passcode keypad number circles in iOS 7, which are in turn also identical in size to the holes in the back of the new iPhone 5c rubber case.
2. The inner part of the new home button is exactly the same size as the circle template for icons (e.g, the circle around the “A” in the App Store icon), minus the exact size of the button chamfer. That is, you need two widths of the chamfered border, plus the icon circle, to be precisely the same size as the outer part of the home button.
3. The microphone hole on the back of the phone is now a perfect circle instead of an oval.
/via Daring Fireball