Comparative neuroanatomist Suzana Herculano-Houzel has done some potentially relevant work. She's figured out a way to count neurons, and has been looking at different species. The upshot seems to be that we're not special as primates go - but primates are special: most species' brain mass scales up and down according to the mass of the animal (as you'd expect, elephants have larger brains than we do) but for non-primates, neurons get larger as the brain does - and what matters for mammals is the number of neurons in the cortex. Primates also scale neuron size, but more slowly (different ‘power law’). Humans have evolved large brains, probably because cooking has afforded us the 'energy budget' needed. Herculano-Houzel's written an interesting and quite readable book: "The Human Advantage".
At the other end of the scale, Michael Levy at Tufts has shown 'intent' all the way down to single cells. That's not 'sentience', of course, but there seems to be a gradient. For instance, a single cell has a 'time horizon' of maybe half a second - and humans often have one longer than their own lifetime. Levy has published a number of studies, easiest way to vind them is via the Levy Lab at Tufts - and a number of his presentations have made their way onto YouTube.
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