Buckle collar:
To measure your dog collar size: take a flexible measuring tape and place it around middle of the neck (or where you place it on the neck) placing the width of two fingers (not flat but pointing out like a scissors) between the tape and the neck. You can also use a string and tranfert on a ruler.
This is the size of collar you need
-The buckle collar measure is from tip of buckle to middle hole. You can also measure a used collar from the inside tip of the buckle to the hole you use (this is the best way to determine size).

Chain martingale collar and/or slip collar:
Take the measurement of the dog head at the largest area in front of the ears, exact, no loose, this is the size you need (you can take a larger size if you preffer).
Chain martingale measure is inside circonference with chain open, the chain open/close on 3.75 inches. Take note that brass chain will tarnish white hairs if leave on dog for long periods.

Here you can see how to measure head properly: