If you have read Eragon, then you know of the Werecat: a race that can shapeshift between a 'large cat' and a 'child', almost certainly with no change in mass. The Magic System is based on the literal and the scientific, thus matter and material cannot be generated or removed without understanding how that could be possible.
I'd like to take this concept and evolve it into what might be considered a 'Kitsune'. Rather, let's say someone magically evolved a werecat into a reflection of a 'Kitsune'. A Werefox for convenience, but not necessarily a race that is part-fox, just somewhat mirrorsing the theme.
A mostly(or only) female race that could shift between a cat/fox-like animal with a long fluffy tail, a human with animal ear+tail features, and a human - but the mass has to be retained. Their evolution should require them to retain as small a figure as plausible, but be capable of safely 'mating with a human to produce children, which would need them to also have a pleasant appearance(if short?) to men in a medieval-type society. Probably closer to a physically attractive woman on a smaller scale in overall size rather than just being shorter? As a (probably unnecessary addendum) they would also prefer running (and probably walking) on their toes when in human form unless carrying significant weight(or would this be a lie?).
Specifics can be changed if it doesn't fit realistically, but this is the general outline. The only magical component to this is the intrinsic ability to shapeshift between working forms. As they would exist in a medieval-type society, the Humans there would typically be malnourished and generally shorter, and they might naturally reflect that if needed.
I would greatly appreciate numbers for both the werecat race and the custom race for comparison. Height: Human(-ears, +tail-mass), Hybrid(+ears, -tail-mass), Animal - also werecats too? Weight: how much (stays the same for each height) - also werecats too?
I'm mostly looking for general height and weight, but details are appreciated.