There are 2 different international measurement metrics known as
Imperial measurement is used by the United States of America, Myanmar and Liberia.
Metric measurement is used by the rest of the world.
The measurement of your product should be straightforward. It is the measurement of the item without the packaging.
Imperial (widely known as ) American System
This system uses inch, foot, yard and mile for measurements of length. The U.S. is the only industrialized nation that has not adopted the use of the metric system.
12 inches is one foot, 1 yard is 3 feet or 36 inches and 1 mile is 5,280 feet or 1,760 yards.
The meter is the basic unit on which the metric system is based. Because the metric system uses a standard conversion factor, it is usually considered easier to work with. Ten millimeters equal a centimeter, 10 centimeters equal a decimeter and 10 decimals equal a meter.
1 inch is 2.54 centimeters
1 foot is 30.48 centimeters
Don't overthink on this. Use your local metric system for the country you live in on your product page.
Some manufacturers write product description length x width x height whiles others width x length x height, but you wont have to worry about this either as it will be outlined on the product page and when adding a product in your dashboard.