Do UFOs have to be human-scale in size?

So this raises the questions related to the "amount" of gravity and the "development" of life... into creatures with the body types-features to interact and modify their environment... develop tools and so forth... and an environment with "raw materials" for tool making and so forth.

It raises a humungous number of related issues....which is one of the reasons I support the 'rare Earth' hypothesis. The number of inter-related steps required for us to become a space faring civilisation is huge. People have this crazy idea that all one needs is a planet in the Goldilocks zone with some water and the right elements...and the Klingons emerge.

Our aliens need an atmosphere with oxygen, because that is required for of the most basic tools for civilisation. An advanced technological civilisation would never arise on Titan, because though there are lakes of hydrocarbons there is nothing to cause oxidation and thus burning. If there was oxygen on Titan you could ignite the entire planet ( which would be rather fun to watch ).

Aliens would need some equivalent of trees, or they'd never get to build houses or ships. They'd need vast periods in which hydrocarbons were laid down, to create some alien equivalent of coal or oil. They'd need an atmosphere thick enough for flight, but not so thick that it behaved like a liquid and made movement difficult.

And so on. We are here as the end result of thousands, possibly millions, of factors being just right. Personally I doubt there's any aliens within 10,000 light years...or even in the entire galaxy.
Not sure if this was what you were looking for!

Yes, but one of the biggest variables is planetary density. It's the reason Jupiter has 300 times the mass of Earth yet only 3 times the 'surface' gravity. When it is argued that up to 10 Earth masses could allow space flight, one has to take into account that surface gravity is related not just to planet mass but to distance from the center of the planet. So, 10 Earth masses could actually have the same surface gravity as the Earth if the planet was large and the overall density was low. Whereas a much smaller planet than Earth could also have the same surface gravity as Earth if it were much more dense.

So....I guess there really is no answer until we find out just exactly what sort of planets are out there. Super Earths do seem to predominate, but I suspect that is largely due to sensitivity of Kepler, and it not being so able to detect smaller worlds that just end up as 'noise' in the data.
How much gravity does a planet have to have before it is simply impossible to take off from it using a chemical rocket ?
If we assume that intelligent life could emerge on a super-earth, then the environment would be very different. A planet with extreme gravity (more than 1.5 gee) would retain much more hydrogen, so the atmosphere would probably consist mostly of hydrogen and helium. This would likely rule out an oxygen-based biochemistry, since any free oxygen would become water very quickly. Respiration on a hydrogen-rich superearth would not resemble Earth-life very closely.

On the other hand, an advanced civilisation that wanted to leave such a planet could use nuclear rockets, such as the Project Orion concept that should be able to launch moderately large payloads to orbit from almost any conceivable rocky world.
The unprecedented extreme power requirements for doing so would be met by nuclear explosions, of such power relative to the vehicle's mass as to be survived only by using external detonations without attempting to contain them in internal structures.
it's probably more likely that an intelligent/crafty/tool-making species would evolve that's x orders of magnitude smaller than humans
I don't know...
...if we're talking about individually intelligent creatures (not collaborations of many small organisms that somehow collectively manifest intelligence) there might be biological constraints on a lower size limit.

Of course, we're the the only prototype of a technological species that we know of, and we only have the example of terrestrial life.
We have no idea how similar or different extraterrestrial life might be. As @Ann K has posted elsewhere, it's a fair initial assumption that life is likely to be carbon-based due to the large number of "useful" molecules that carbon allows, and its abundance.

On Earth, all nervous systems are built with neurons. There's much interest in recent findings about how fungi might "communicate" through long sub-soil hyphae, and possibly trees "cooperating" via root systems, but I'm sceptical that this level of interaction, with very slow signalling (compared to nervous conduction) would enable an intelligence capable of tool construction.
-But I might be biased by the lack of examples of such intelligences on Earth!

If ETI have central nervous systems, or an equivalent, based on similar chemistry to our own, I wouldn't be that surprised if the essential signalling/ processing components in their organ(s) of intelligence were similar, or at least analogous, to neurons.
On Earth, there is variation of neurons between species, but no alternative to the neuron has evolved.

It doesn't take much reduction in size from the lower end of healthy human brain sizes to start having profound effects on intelligence. And our large brains require a lot of oxygen and glucose (or alternatively ketones), requiring substantial respiratory, cardiovascular and nutritional systems- and they all require other systems to support and protect them.

(I admit this reasoning might be very human-centric, maybe smaller organs of intelligence with smaller, or more efficient, components could exist).

You seem to be discriminating against hive minds here.
The most Woke phrase I've ever read! :D
Your postulated small guys would require no competing species.
I don't get this. We survived despite there being predators larger than us, and most extant species are smaller than us, despite having predators/competitors larger than them. Further, if we entertain the trope of replacing our biological bodies with technological bodies, I see no reason why those bodies would need to be human-size.
A technological body could be almost any size, depending on the required function of that body. Large artificial bodies would be useful for lifting and carrying materials, smaller ones better for fine work and perhaps better for covert observation. It would be more difficult to hide a dinosaur than a wasp.
think you sort of missed the point. Our planet has over 10 million species and probably had another 50 million in the past. And out of all of those only ONE ever made it into space. The hominid shaped one with opposable thumbs.
With due respect, I think you are trying to make a lot of stew with only one oyster. Plotting a curve from one data point is a bit tricky.

Give me an octopus with a long lifespan that lives on land, and some time, and well see what happens... but I don't see any reason they could NOT make it into space, non-humanoid though they be.
Give me an octopus with a long lifespan that lives on land, and some time, and well see what happens... but I don't see any reason they could NOT make it into space, non-humanoid though they be.
Does that octopus evolve humanoid form and opposable thumbs? If not, you can forget it, buddy. It's either develop humanoid morphology and opposable thumbs, or suck on sea slugs for all of eternity. The Space Shuttle proves it!
If your contention is that tentacles cannot be as dextrous as a hand, we may just be in an "agree to disagree" position.