In almost every situation your clip-on frames only come in one size, so as long as the model numbers match it is the correct one for your frames.

It helps to remember that your frames were most likely adjusted to fit to your face, or were changed bit by bit over time as you wore them. Your clip-on is arriving as fresh as it left the factory! You will probably have to do some adjusting to make them align perfectly. Carefully bend the clip-on frame at the bridge and the sides to help the magnets and lenses line up with the current shape of your frame. If the clip-on is plastic-based, you might need to heat it with a blow dryer or similar hot air machine before manipulating it.

If you are not comfortable doing any of this yourself, you can take the glasses and clip-on to your local optometrist and they should be able to help you.