Add a measure
If your shoppers wonder about the size of your products, adding a size comparison image is important.
In this example, they sell plants, and put the plant side-by-side with an adult, to give you an idea of its size.
Add a person
If the comparison is a person, you can mention the height of the person.
In addition to a comparison image, you could add an in-scale image (in context). See best practice #84.