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 (I assume you want them to be mammals?) is the Etruscan shrew.
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) & others to its question, you may find they give you some ideas.