Corvids in general, and in particular Crows, are fiendishly smart. They recognize faces (though I don't think they recognize their own reflection, but not sure). They pass knowledge on to other crows. They solve multi-step problems without resorting to trial and error. They make and use tools. They are undisputedly some of the most functionally intelligent species on Earth.
This is fascinating because their brains are, quite literally, bird brains. They do not have a particularly high Encephalization Quotient. They do not have very large brains overall, in fact. Their brains are just more special than, say, a small monkey.
Due to the size of animal cells in general having a lower bound, doubtless there is some lower bound to brain size which will end up restricting how small you can get before you just cannot be as intelligent as a human. But if Crows are skirting that line, a half/pound mammal would also be.
And that's just considering the intelligence of the individual. Have you read Vernor Vinge's A Fire Upon the Deep?