I agree that a scale is very helpful - not necessarily an actual person, though. I don't know what something will look like on me if it's modelled on someone twice my height and half my weight (-:
M & S, a very big clothes seller in the UK, says how tall the model wearing a dress / trousers / skirt etc is. That's very helpful in working out the size of something.
So someone's neck, or hand, or wrist is great. But not essential, I reckon.
For example, in this one:
http://www.marksandspencer.com/Auto..._1_46&nodeId=527058031&sr=1-46&qid=1331603751
They tell you the model is 5 ft 10 and size 12.
Back to jewellary, and I think this site does it rather well - they use attractive, but not model-type, women, and concentrate the photo on the relevant part;
I also think detail's important. So this item pictured here has 4 photos, including this one:
And is also fairly clear about what the item is:
"This smoky quartz and Swarovski crystal pendant is on a 9ct yellow gold 20inch hammered trace chain for a modern look. The stunning smooth smoky quartz pebble is 27mm long by 20mm wide and is set off by two further drops of golden brown coloured Swarovski crystals and small smoky quartz spheres. The entire pendant measures about 85-90mm from top to bottom. This fashionable pendant would look great worn over neutral coloured tops or could be teamed with a low neckline."