The following chart refers to all personalized collars in http://www.furrygems.com/personalized-collars/
Important Sizing Note: The measurements all of size chart are the dimensions of the actual product, not of the pet they fit. After measuring your pet, make sure most measurements fit within the following dimensions especially the chest, then add 1-2 inches for a comfort fit.
Note: Each item has its own size chart, please measure accordingly
There are different sizings ranges for different types of garments, it is due to the binding or flexibility of the materials. If you are ever stuck between 2 sizes and are not quite yet to the next size, then choosing the bigger size is always better.
The most important part is the chest. If the chest size fits properly, then everything else will not be a big matter.