Sizing depends on the style collar you want to order.   See my previous article on collar styles then follow the measuring instructions below.

Use the featured photo as a visual measuring guide:

For collars that sit high on the neck:

1. Use a cloth measuring tape (tailors or sewers tape measure) for best  accuracy. 2. Lay the tape around the neck mid way between the jawline and the  hollow of the throat. A. Don't pull the tape tight but rather ensure it lays flat against the  skin all the way around the neck. B. Don't leave any slack and ensure you don't have a finger or thumb  between the tape and the skin. 3. Add .5 inche to the resulting measurement to get the proper size to  order for this style.  

EX. 13 inch neck at level of Adam's Apple gets plus .5" means you  should order a 13.5 inch collar.  

For collars at the base of the neck:

1. Use a cloth measuring tape (tailors or sewers tape measure) for best  accuracy. 2. Lay the tape around the neck mid way between the jawline and the  hollow of the throat. A. Don't pull the tape tight but rather ensure it lays flat against the  skin all the way around the neck. B. Don't leave any slack and ensure you don't have a finger or thumb  between the tape and the skin. 3. Add 1.5 inches to the resulting measurement to get the proper size to  order for this collar.  

EX. 13 inch neck at level of Adam's Apple gets plus 1.5" means you  should order a 14.5 inch collar.  

For Chain style collars:

Follow the instructions above and then:

Add in 1.5 inches for base of the neck wear

Add in 2 inches to sit just below the hollow of the throat.

Add in 3 inches for a standard necklace length

Anything longer than this is purely up to you.  Here's a general guideline to all necklace lengths and names.  Keep in mind that the actual neck measurement will impact fit on all standardized sizes:

Collar 1/2 inch larger inner circumference than throat measurement.  Sits at the midline of the neck.

Choker 35 centimetres (14 in) to 41 centimetres (16 in) long and sits  just around the base of the throat.

Princess  A princess necklace is 45 centimetres (18 in) to 50 centimetres (20 in) long, 

Matinee A matinee length necklace is 56 centimetres (22 in) to 58         centimetres (23 in) long - typically a single strand that rests at the top of the. 

Opera An opera necklace is 75 centimetres (30 in) to 90 centimetres         (35 in) long and sits at the breastbone. 

Rope A rope necklace is any necklace longer than opera length. 

Lariat  A lariat is a very, very long variation on the rope, without a clasp, often worn draped multiple times around the neck; the  ends can be crossed over, looped, or knotted in various ways.  This type of necklace sometimes incorporates a loop at one or  both ends to allow it to be worn in the style of a lasso or it may be worn doubled over with the ends passed through the         loop formed in the middle. 

Riviere-Gemstone necklace single strand all stones same size. 

Above information obtained from Wikipedia.