What term is used to describe the thickness or size of a material, such as a shotgun shell?

The term that describes the thickness or size of a material, such as a shotgun shell, is "gauge." In the context of firearms, particularly shotguns, gauge refers to the internal diameter of the barrel and the corresponding measurement of the shotshell that fits into it. A lower gauge number indicates a larger diameter, and this system is widely recognized in the industry.

Caliber is a term more commonly associated with rifled firearms, referring to the diameter of the bore of a firearm barrel in inches or millimeters. Diameter itself is a more general geometric term used to describe the distance across a circle or cylindrical object, which does not specifically pertain to the firearm industry terminology. Volume, while related to measurement, does not accurately capture the specific concept of gauge as it pertains to shotguns and their shells. Therefore, gauge is the correct choice in identifying the size descriptor for shotgun shells.