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.
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 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, 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 for a larger than the 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 (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.