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Assume that an external force nudged along the evolution of a mammal and/or insect species towards an appearance mimicking as much of humanity as possible - skin, mouth, language, fingers and thumb, etc - while capable of flight (like a hummingbird? or Tinkerbell). Assume that their minds are built and wired differently to allow higher intelligence, if needed.

Would there be a size they could be viable as a species capable of flight (including hovering)? What would be the range of height and weight for such a species? How would their biology differ? Diet? Would they be capable of human-like speech or would their speech be different?

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I'm assuming you mean 'how small' could they be?

For the viability of a humanoid body plan we can look to the pigmy marmoset.

Smaller than that may be possible but clearly 'as small as' definitely is.

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Could something that small speak? yes, ask any budgerigar.

Can they fly at that size? lots of things that size fly.

It's just a matter of giving them adequate wings & appropriate musculature to drive them.

The smallest mammal by weight (I assume you want them to be mammals?) is the Etruscan shrew weighing about 1.8 g (0.063 oz) on average with a body length of about 4 cm (1.6 in).

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The bumble bee bat (a flying one for you) is about 29 to 33 mm (1.1 to 1.3 in) long & 2 g (0.071 oz).

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So I see no reason you couldn't go down to their size (though as there's nothing smaller than the Etruscan shrew that's likely your smallest reasonable limit for a mammal).

Their voices may be a bit distorted & high pitched that small but I see no reason they couldn't still talk.

How would their biology & diet differ? shorter lifespans, higher metabolism, faster heartrate & having to feed far more frequently are strongly indicated by our knowledge of small mammals.

Intelligence is a whole other ball game, human intelligence in a hard science context for something this small is not really possible, with a bit of handwaving you might get away with equivalence to a Three year old toddler for a larger (than a smallest possible) model without too much grumbling from the pedants, but the smaller you go the softer the science & ever more handwaving needed I fear.

Have a look at this answer (you've reminded me I haven't finished tidying it up, thanks, I'll have to get onto that) to a related question & other answers to it, you may find they give you some ideas.

Later edit: I'm 100% satisfied with my conclusions on size but not entirely with those on intelligence.

In tests a Raven does roughly as well as a Chimp & both are considered roughly equal to a two year old, they do better in some areas but broadly speaking across all tests that seems to be the consensus on where they average out.

But the Raven does it with more or less 4% of a Chimps brain mass (about 15g), we think they achieve this with a greater neuron density than in mammals.

Extrapolation from that suggests a 54g brain (assuming the human average of 2% of body mass that's somewhere between a one foot six inch & two foot tall person I think? meh! someone else will have to do the math on that for you) using the same tricks birds do to cut brain weight (to make flight easier) could plausibly be as bright as an adult human.

But I'm far from convinced it can simply be scaled like that to produce human level intelligence & the danger is new work in the field could potentially make any such extrapolation appear silly tomorrow or that there already is work out there that does I've just not come across.

You need an expert in the area for that one.

You'll find other answers on size & intelligence in this question.

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  • $\begingroup$ "How would their biology & diet differ? shorter lifespans, higher metabolism, faster heartrate & having to feed far more frequently" - make them brightly colored and adopt their mouth to be able to reach nectar in plants ... wait. $\endgroup$
    – John Dvorak
    Commented Sep 1, 2021 at 12:36
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    $\begingroup$ @JohnDvorak Plausible for a small animal that needs a regular high energy boost but somewhat breaks the OPs criteria that they should have an appearance mimicking humans (baring the wings, faeries have wings & the OP wants those) // you might get away with a tongue for it (rather than a long proboscis or beak) that doesn't break the overall aesthetic but then they won't be able to talk which the OP wants, & there are plenty of small animals that survive without nectar so the frame challenge there doesn't seem necessary // anyway, they've hands, can be tool users & get at it that way :) $\endgroup$
    – Pelinore
    Commented Sep 1, 2021 at 13:25
  • $\begingroup$ To add on to the intelligence question, my first thought was that the smallest animals I'm aware of that display many human-like behaviors are birds like crows, ravens, and parrots. So I don't think it'd be too handwavy to give them nearly-human-level intelligence at that size. $\endgroup$
    – hobosullivan
    Commented Sep 2, 2021 at 16:19
  • $\begingroup$ @hobosullivan birds have a few tricks up their sleeve other than overall mass & complex topography (all those little folds & wrinkles) that we use, they've found ways to achieve greater neuron connectivity & have (on average) smaller neurons iirc (been a while since I read the articles), the only truly bright birds out there are among the larger ones though & only approach a two year old toddler at best. $\endgroup$
    – Pelinore
    Commented Sep 2, 2021 at 16:26
  • $\begingroup$ @hobosullivan ^ for something smaller than a crow, raven or parrot I wouldn't expect anything better than a two year old toddler even if they combine birds tricks with ours (I'd expect the extra benefit of that combining to be countered by the loss in size), down at the minimum size (the Etruscan shrew) you'd be lucky to do any better than an ordinary budgerigar even with all those tricks I think // but this is fantasy & someone else's story, the amount of handwaving they want is up to them :) $\endgroup$
    – Pelinore
    Commented Sep 2, 2021 at 16:42
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A 4.6 g hummingbird has a wingspan of 117.5 (4.6 inches). For an average human female of 159.5 cm (5'2") tall and 53 kg (117 lb) to fly like one, it would need a wingspan of about 12.6 meters (41.4 feet), which is very impractical and wouldn't work. If it was much shorter, say 50 cm (1'7") tall and 1.6 kg (3.6 lb), it would still need a wingspan of 2.19 m (7.2 ft). If it were only 85 grams (3 oz) and 18.7 cm (7.4 in), then it would only need a wingspan of 50.5 cm (20 in).

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