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 may 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]

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) & others to its question, 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