Piping can be made of copper or galvanized steel, as small as 1/2-inch internal diameter for a recirculation return to 2 inches or more for mains. More typically, residential-tank piping will be 3/4-inch diameter and commercial will be 1-inch, 1 and 1/4-inch or 1 and 1/2-inch.
One of the reasons to have an entry in the glossary of so obvious an item is something not so obvious. We may want you to tell us the diameter of the piping. Be aware that steel will have a thicker external diameter relative to the internal diameter than copper.