What criteria are used to classify metal tubing according to size?

The classification of metal tubing by size primarily focuses on the wall thickness and outside diameter. These two criteria are critical because they directly influence how the tubing will perform under various conditions, such as pressure and temperature. The outside diameter is essential for fitting the tubing into existing systems, ensuring compatibility with other components. The wall thickness is crucial for determining the tubing's strength and durability; it affects how much pressure the tubing can withstand and its overall stability in applications.

Considering the other options provides insight into why they are less suitable. Length is indeed a factor in certain contexts but does not play a role in classifying size; it refers more to the application of the tubing rather than its dimensions. While diameter is essential, it must be specified as the outside diameter, and weight is not a primary classification criterion. Material type is also important but relates more to the properties and applications of the tubing rather than classifying it by size. Therefore, focusing on wall thickness and outside diameter offers a clear and standardized method for classifying metal tubing sizes.