All the smaller animals - mice, rabbits, hedgehogs, and the like - are all a bit bigger than their earthly counterparts. That comment is made somewhere, and is explicitly described when Aslan grants the gift of speech to the chosen animals in Magician's Nephew. I gathered from the illustrations and descriptions that Reep would be about the size of a large (not huge) cat, though in the form of a mouse. Talking rabbits and moles would be about knee-high. The larger talking animals, such as elephants, were a bit smaller.