These terms relate to the product offering by a company, especially if the company manufactures a large number of different products (including similar products).
Product Line: products of similar nature, price, function, intended market, etc. form a product line. It is different from product bundling where similar products are bundled and sold together as a unit. A product line comprises similar products that are sold separately. A company can have a number of product lines. For example, Toyota has a product line of sedans, a product line of SUVs and a product line of pick-up trucks. In the product line of sedans, Toyota offers Corolla, Camry, Avalon, etc.
Product Length: refers to total number of products sold within a product line. For example, Asus may have 10 different types of netbooks.
Product Width: refers to number of separate product lines offered by a company. For example, the product width of Toyota may be 3, one each for product lines of sedans, pick-ups and SUVs.
Product Depth: refers to the number of sub-categories within a product line. For example, Adidas may have 5 different sub-classes (say basketball shoes, athletics shoes, tennis shoes, etc.) within the product line of sports shoes.