When shopping in a physical store, it’s easy to judge the scale of products. Online, it’s much harder.
According to Baymard’s usability research, 42% of users try to estimate the product’s size from its product images. They also found that 56% of test subjects’ first action upon arriving at a new product page was to begin going through the product images, before reading titles, descriptions, or scrolling down the page. (source)
For example, show the product within a typical environment, or people interacting with the product. Include other products or objects to give shoppers a sense of its proportion.
In addition to having an in-scale / in-context image, you could also add a comparison image (see best practice #180)