**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 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 but you haven't asked about that.*

-----

Have a look at [this answer][6] (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://worldbuilding.stackexchange.com/a/158515/49261