Length is considered an extensive property. Extensive properties are those that depend on the amount of matter present in a system. Since length measures the distance or extent of an object in space, it will change if the amount of the material changes, such as if you were to cut a rod in half. The two resulting pieces will each have their own lengths that add up to the original length.
On the other hand, intensive properties do not depend on the quantity of matter. Examples of intensive properties include temperature, density, and color. These properties remain the same regardless of how much of the material you have. For instance, the temperature of a substance will be the same whether you have a liter or a milliliter of that substance.
In summary, since length varies with the amount of material present, it is classified as an extensive property.