The prong collar is a collar that needs to be fitted snug on your dogs neck and fitted as high up as possible. The reason for this is that the pressure points of the collar need to be sitting against the skin so that only a small amount of effort is applied to the collar to effectively give a correction.
The other reason is that prong collars are sized by adding and removing links. The links are held together when the collar is kept taught, not loose.
You can use a flexible tape measure to wrap around your dogs neck in the location that the collar will be fitted. As mentioned earlier, as high on the neck as a collar can be fitted.
Once you have this measurement, on each individual prong collar product page, there are the measurements at which the collar is supplied standard and how much length each extra link adds or subtracts. For example, if you were choosing the Herm Sprenger Stainless steel collar, and your dogs neck measures 50cm, the medium collar supplied at 57cm will be the correct size, you will just remove some links to fit.