(L) Most dog coat brands base the size of their coats on the length of the back bone of the dog, simply measure from the base of your dog's neck to the base of their tail. This is more about preference, just be sure to cover your dog's core.
(C) The second measurement is the girth which would be measured around the widest part of your dog's chest, usually just behind the front legs.
(N) And the third is the neck measurement. You should measure all the way around the lower part of your dog's neck where the coat collar would sit.
A good rule of thumb to test for snugness is to leave enough room for two fingers between the coat and dog. If you are between sizes then select the bigger size for comfort!