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.
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).
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).
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 probably your smallest limit.
Their voices may be a bit distorted & high pitched that small but I see no reason they couldn't still talk.
Intelligence is a whole other ball game but you haven't asked about that.
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.