Everybody has a concept of what inches and feet or meters and centimeters measure like. But we don’t think of sizes that way. When we see a sofa, we don’t think of it as an object that is four, five, or even six feet wide. We think of the sofa as a three seater or a four seater.
The same method of determining size by relative comparison is continuously and subconsciously employed by our minds on other objects. Take towels for example. Narrow towels are preferred by men and are usually wrapped around the waist. Women prefer wider towels. These can be wrapped from just below the arms and cover all the important bits from the chest area to the knees.
Saying a towel is six feet long and four feet wide does very little to stimulate the mind’s eye. Showing the picture of a model wrapped in that particular 6’ x 4’ towel and covered from chest to knee communicates a much clearer image of just how large the towel is and that message is immediately grasped by our brains.
Product comparison images allow shoppers to quickly and easily determine the size of an object without resorting to mental conversions or complex visualizations.
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 includes a person, you should 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.