The "O Scale Classic" items by K-Line are scale size. Many of them are generic designs that are not modeled after a specific prototype, but they are scale size, not traditional size. The older molds, such as the K761 series boxcars and the K762 series reefers, do not have a lot of detail but on the other hand they are quite rugged if you are transporting them back and forth to a club layout. Newer K-Line scale items such as the K742 series reefers and the coil cars are nicely detailed scale models.
Also note that the Heinz closed vat cars, which are listed on the box as "traditional size," are in fact scale size. I once measured one against a scale drawing and it came out as the correct size. The prototype cars were small, maybe 36' or so.