**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. [![enter image description here][1]][1] 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][2] weighing about 1.8 g (0.063 oz) on average with a body length of about 4 cm (1.6 in). [![enter image description here][3]][3] The [bumble bee bat][4] (a flying one for you) is about 29 to 33 mm (1.1 to 1.3 in) long & 2 g (0.071 oz). [![enter image description here][5]][5] 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][6] 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][7] (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. [1]: https://i.sstatic.net/t3THC.png [2]: https://en.wikipedia.org/wiki/Etruscan_shrew [3]: https://i.sstatic.net/BBU0a.png [4]: https://en.wikipedia.org/wiki/Kitti%27s_hog-nosed_bat [5]: https://i.sstatic.net/p4mcM.png [6]: https://raisingchildren.net.au/toddlers/development/development-tracker-1-3-years/2-3-years [7]: https://worldbuilding.stackexchange.com/a/158515/49261