How to Determine the Right Size For Your Dog Collar
A leather dog collar is strong and durable and with proper fit and care can last much longer than mass produced nylon collars.
Getting the right size is incredibly important. Here are my suggestions for getting a correct collar measurement.
Does your dog have a collar currently? If so, measuring length of the old collar is easiest way to get a proper size for ordering your new collar.

Lay the existing dog collar out on table and place end of measuring tape at the point where the leather, nylon, or other collar material folds at the buckle end. Then measure the collar to the point where the dog currently wears the collar (if it is a buckled collar) or to the fold at the opposite end (if it has a quick release buckle). This is the measurement I use to make the collar and I leave at least 1.5in on either side of that measurement for the collar to adjust smaller or larger.

For collar width, take into consideration your dogs ability to move and eat when wearing the collar. The majority of Small to Medium sized dogs will be most comfortable with a 3/4in collar. Very large breeds can wear up to 2in collar. Again, if your dog already has a collar, sticking with the same width will be the most comfortable for your dog.
If you do not have a current collar to take measurements from, I suggest the following site:
Simple Ways to Measure Dog Collar Size: 9 Steps (with Pictures)