Nowadays, the number of people who prefer to make online purchases on e-commerce platforms is constantly increasing. Online shopping turns out to be fast and cheap but it also involves some risks. In fact, it sometimes happens that a product, once received, is smaller or bigger than expected, or does not comply with what is described on the purchase site. Typically this happens because the information provided by retailers is not detailed and, moreover, the images usually show the product on a white background without any reference nearby to understand its true dimensions. To cope with this problem, we developed CompareDimensions, a software tool that helps customers during their online purchases, by allowing them to understand the correct dimensions of the product they are about to buy. Through this tool, a customer can see the images of the product and compare it with a series of reference images known to all, such as a man, a chair, a door or a child. The experimental evaluation, conducted on Amazon, shows that the use of the proposed tool helped people to figure out the real dimensions of products, with respect to the case in which only cut-out images and textual descriptions are provided. In particular, according to our experiments, people who used the tool were able to understand the real dimensions of a product with 17.5% greater accuracy than those who did not use the tool. Focusing on critical products, average accuracy increased by 24%, with an improvement of 27% and 11% for products that proved to be smaller or larger than expected. Therefore, the adoption of CompareDimensions could help customers to understand the real dimensions of the products they want to buy, and consequently reduce the likelihood of a return.