Yes, combination wrenches come in a million different sizes that are all clearly labeled as to what size they are, in standardized units, with either end of the wrench giving you a visual scale of the labeled size, making it easy to determine the scale of an object.
It would be pretty simple to resize that image so the opening of the wrench was 15mm on whatever screen you were viewing it on, and measure the spider accordingly. This is why it makes a good scale.
Yes, combination wrenches come in a million different sizes that are all clearly labeled as to what size they are, in standardized units, with either end of the wrench giving you a visual scale of the labeled size, making it easy to determine the scale of an object.
It would be pretty simple to resize that image so the opening of the wrench was 15mm on whatever screen you were viewing it on, and measure the spider accordingly. This is why it makes a good scale.