Buying pet collars online isn't as straight forwards as it seems..
The only difficult part of purchasing a collar is the size, many different sellers will sell a M collar as a L and a S as a XS. So it's always best to see what the seller has written in the description as here there is usually more detailed information on the size.
Usually in the description of a collar there will be a more accurate size guide which will look something a little like this: 6-8". This example means the collar would fit a pet with a neck circumference of 6 to 8 inches. Adjustable collars are slightly different and most likely won't have a numerical example in the description and will usually stick to the S, M, L scale - maybe giving a breed or two as example sizes.
To measure your pet's collar size, take a tape measure and LOOSELY put it around the animals neck, with a collar you should be able to fit two fingers between the collar and neck of the animal. So you should use the tape measure in the same way. Take note of the measurement and then you will be able to easily shop online for a pet collar.