The official decals of Ford's Special Vehicle Registry, they measure 2.75" in diameter and are reverse adhesive so that they can be applied inside of your vehicle's windshield*.
Depending on your Mustang, it's possible to order multiple decal designs. For example, you might have a Mustang FOX, which is also an SVO or a 1968 Shelby GT500KR, which is powered by a Cobra Jet. Of course you can order the classic Mustang registry decal!
* Make certain to apply this decal in an area, which will not obscure your view when driving and comply with motor vehicle laws and regulations.