Encouraging a sense of scale in photos

[obscurus]

I’m often frustrated by the lack of a scale in photos, too. However, I also remember a discussion here about the way annotations to a photo (such as a circle or arrow to indicate the organism of interest) were discouraged because they can affect the CV learning process. Is there a risk that the inclusion of a coin / lens cap / spotted finger / thumbnail / scale ruler in a photo could do the same? I would hope the benefit of including a scale would outweigh that, but who knows.