Problem with that is that if you looked at a picture of a giraffe in isolation, and an elephant the same, you wouldn’t necessarily realise that the Giraffe is significantly taller than the elephant.
The giraffe has a thin body and thin legs. The elephant has a fat body and fat legs. I’ll bet that if you computed the leg stress (i.e., psi or pascals) of both animals, they’d come out similar.
That said, you would want to use any obvious anatomical traits to correct the result as much as possible. Elephants don’t jump, and it’s fairly obvious from the way they move that their legs are under more stress than much smaller creatures like cats.
I should add that when I say “roughly” I mean “within an order of magnitude”. Humans and elephants are roughly the same size. We’re both much bigger than bugs, which themselves are all roughly the same size.
If it comes to that, you could just ask them the size of their males’ genitalia. Substract about 25% off the number they give, and you’ve got a precise scale for the picture ! 
Another thought: if we can make any request about the composition, I suggest the alien take the picture on Earth next to a height chart. 
This was discussed in a documentary I saw on TV when I was a kid. I loved that show. The relevant part starts around the 40 minute mark.
Another reason why they might be generally the same size as us; fire - it doesn’t scale up or down linearly. A creature the size of an ant will not be able to make a fire hot enough and sustainable enough to smelt metals to make tools. A giant would probably be able to do it, but not at a giant scale.
Tell him to use camera lens radius as unit of measurement, and ask him the distance of the lens from sensor, sensor size, and the distance of himself from the lens.
Meter is not a natural physical size. but atom size or emission wavelengths are.
We can already get the distance to him in terms of how big the lens is. But how can we tell how big the lens is?
Wouldn’t fire be affected by gravity and atmosphere?
[Translated from the alien language]The humans must be our size. Obviously sentience can only occur with silicon-based crystalline organisms and within a nebula’s microgravity. The surface of a planet would have gravitational fields that scramble the positronic pathways before intelligence can evolve naturally.
Another reason why they might be generally the same size as us; lasers - it doesn’t scale up or down linearly. A creature the size of an quatloof will not be able to make a laser sufficiently intense to make computation arrays. Just think how clumsy the appendages of a blazmoort would be in trying to manipulate laser light.
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As I said–“if we limit ourselves to beings that we even recognize as intelligent life”. Life may well be really weird. Is that crystalline entity living or not? How could we know? How could it evolve in the first place?
There are actually good reasons to think that aliens are probably similar to us in composition. Interesting interactions happen in all kinds of physical environments: plasmas, nuclear reactions, Bose-Einstein condensates, etc. But nothing we’ve found so far compares to the raw richness of chemical interaction. Maybe we’re biased, but it’s not like we don’t have a pretty broad understanding of this stuff. And in the realm of chemical interaction, carbon is far and away the most flexible building block.
You’re already limiting yourself to a narrow slice of imaginable life by positing that a photograph (what is a photo to, say, a computer program?) or indeed any communication is possible. It’s not too much of a further stretch to guess that aliens that we would recognize as life would have evolved on a planet with an atmosphere, use EM radiation as their primary means of sensing the world, and that they are chemical in nature.
Sure, we can go crazy with our imagination, but your question doesn’t make sense unless we make some simplifying assumptions. Then again, we could just say “No.”
PS: You specified “home planet” in the OP. I don’t see why you’re dismissive of us assuming a planetary surface.
If the creature is exhibiting the alien equivalent of “fish lips”, we can conclude that it lacks intelligence as we understand it.
Well, aren’t we unsure about photosynthetic life as opposed to …?
Also, why is the record of the length of time for life to evolve on Earth a standard that should hold in another similar (for the sake of argument) environment for photosynthetic (for the sake of argument) life? I’ve seen similar comments and I never know if they make sense. (Not knocking Dr., just asking).
Um, is this life in our Universe? That’d be be some camera. Also time and position could get you the other information, but again, can’t be done.
Well, obviously it’s hard to guess how exactly alien life would make use of their star’s light. But we can guess that it’s not too far off from Earth life.
A factor of 10 reduction in wavelength puts the light into the deep UV. UV light is highly damaging to chemical structures (both biological and not) and won’t even make it through many atmospheres (N2, O2).
On the other hand, a factor of 10 increase in wavelength puts light in the deep infrared. Infrared light doesn’t do much beyond heat things up, its hard to make use of for any chemical process, and just doesn’t contain a lot of energy. It also doesn’t seem like a good fit for the alien equivalent of photosynthesis.
What we consider visible light is a good balance between these extremes. The photons are energetic enough to drive chemical processes, but not so much that they destroy everything else. It’s not really a coincidence that photovoltaics also run mostly on visible light–many energy-harvesting techniques are subject to the same fundamental issues.
Simple answer is that we don’t know. That said, time 0 to photosynthesis isn’t the long pole. That part was remarkably fast on Earth; only a few hundred million years. The hard parts were complex (eukaryotic) cells and multicellular organisms. If some alien world did the photosynthesis part 10 times faster, they would still need a several-billion-year star.
Evolution seems to happen a a fairly steady rate at the largest scales (even if you agree with punctuated equilibrium). It doesn’t seem like we got particularly lucky or unlucky in any respect. If some particularly necessary step were advanced or delayed by a few million years, it still doesn’t change the overall picture.
So if intelliegent life were to arise on a star with a very short lifetime, say on the order of 100 million years, they would need to be much more efficient in their evolution. For that to happen it seems like they would have to be pretty different from us.
Hey, I said it was a high-tech camera :-). At very low photon densities, we can sorta do this now. And it’s actually not that hard at all if we give up on the direction and just want a many-band multispectral camera. Some shrimp can see 12 different frequency bands; maybe aliens can see even more and hence need more advanced cameras.
How about a pot of boiling water?
Just a WAG, but I’m trying to think of a simpler approach using basic chemistry, rather than more convoluted techniques with optics or other physics.
Does boiling water produce steam bubbles of roughly the same size, no matter the size of its container, sea-level or atmosphere? If so, that might serve as a “standard candle,” such as cepheid stars in astronomy.
First of all, I don’t think assuming the aliens have a similar chemistry to us tell us very much. Evolving on a planet with a different surface gravity can vastly affect the size of organisms. I think this can cause size difference of more than an order of magnitude even if we have the same type of DNA.
Secondly, I don’t think steam bubbles will work. For one, you’d want a clear container to photograph, not a “pot of boiling water”. And the bubble size is determined by the equilibrium between the outward steam expansive gas pressure vs. the inward force of the liquid water. The inward force is affected by water pressure, which in turn is dependent on factors such as gravity and atmospheric pressure.