Blowing Conch Shell 6"+
How Blowing Conch Shells are Measured, and Why
Below are 2 images that show how the conch shells are measured. The images depict 6" blowing conches. The important thing about a conch shell and what determines the price is the amount of space the conch shell has inside it for the air to blow around it. They call it the "blowing arch". So you could have two conch shells that are both 6" long and one could be very skinny and one could be very fat. The fat one would have a lot more space inside it for the air to blow around it so it would be more expensive than the skinny one that has less space for the air inside it.
You can see in the photo how it is measured around the blowing arch to account for this. Since conches are created by the natural world, and they are found in the ocean, every one is a bit different.