This is something I did try at one point in my evolution, but found it did more harm than good for people choosing toys to play with, as it often takes the place of looking at the measurements, which is more accurate.
With a global reach, it’s nearly impossible to find an object that can be relatable across all cultures. Take the aluminum can for example, which is an object commonly found in adult toy product pics, there are several different sizes of cans depending on where you are in the world and what was contained in the can. I mean there are a lot of assumptions being made especially if the precise measurements of the can aren’t provided. Pair this with the fact that visual perspective can make the can or other object look smaller or larger relative to the toy depending on where it is placed in the camera’s field of view.
The best course of action is to measure a toy you enjoy so you know where a good starting point is. There are handy sizing charts on my site that you can print and use to place on them where your current collection of toys are and immediately see where the toy you are interested is falls within your current play capabilities.
And if you are looking for your first toy, a good technique is to find an object around the house that is the size of what you think you’d like and measure that to get a sense of the circumference and length you want to target. If you can’t find something exact, you could take a smaller object and wrap it in cloth and tape until it’s the size you want and measure that.