Usually when I take photos, particularly of a natural landscape – I intentionally avoid scaling the picture with human figures.  In a way, I like to play tricks with the viewer and not have a particular reference point.  But every once in a while having a figure to scale something in your photo adds a lot, particularly if you want to show how impressive something is.  Whether a natural landscape or a man made structure.  However, lately, I have tired exploring scaling using non-human reference points.  Typically, it would have to be something familiar to the human mind to be able to make the scaling reference point useful.

1. I use birds and small wildlife quite a bit, particularly to scale countryside natural landscapes.  This way a human does not disturb the quiet calm of the picture and the scale reference is there.
2. For urban photos, I have used boulevard landscapes such as trees to act as a reference point for building massing or built form.  Street level cars are also a nice alternative.  Typically, in an urban setting you will inevitably have human scale as a reference.  Of course I play around with this by have smaller items such as trees in the foreground with the building in the background to give a skewed sense of scale.
3. I have many photos of lakes and larger water features or beaches.  If it is in a more wilderness area I use vegetation such as trees, or if available boats and empty kayaks to scale the endless horizon.  Lately though I have used smaller scale references such as hermit crabs and sea shells to juxtapose with the larger beach/water body off into the far horizon.
4. Sometimes I use similar objects to frame each other, so in an urban city, I would scale buildings by taking an angle that includes another building in the foreground or background to provide a cool looking puzzle or patchwork of built form – usually of windows.
5. I’m not a fan of people who stretch out their arms to provide a scaling reference ot their photos, I would rather leave a shoe on a beach or hang a hat on a tree in the foreground to provide more artistic and indirect scaling references in my photos.
6. Scale, size and references in a photo is all about perspective so the angle determines how something is viewed (and lighting should get equal credit).  Playing with the perspective of the photo is half the fun, whether you take it from the ground level or peaking from a side of a window.  The scale will be affected and how the eventual viewer sees it.

I continue to explore this aspect of photography and it is my new fascination.  Would love to hear if there are any thoughts on this, please add to the comments with some other ways scaling and references can be incorporated in your photography.