The Logistics of Shifting - Is Anything too big or too small?

As I write my shapeshifters I begin to wonder just what the limits are for shapeshifters. Are there any limits that would make a reader go "Huh?" when it comes to shifting into an animal far larger or smaller than human.

Could a human shift into a creature as large as a humpback whale or as small as a mouse? What would stretch a readers belief in your world? Is there a point beyond which writers shouldn't go?

I've mentioned that there are writers that have written about their humans shifting into dragons. Now dragons are darn big - but they are "magical" creatures to begin with. To believe in a dragon you have to suspend your disbelief before you begin. So imagining a human becoming a dragon isn't outside this particular shapeshifter trope.

But what if you write more mundane sorts of animals, like I do? Would someone buy it - literally and figuratively? I've considered expanding my shapeshifter world but when I thought about my humans shifting into something the size of a whale, for instance, it seemed too big. How does a normal human male (weighing approximately 200 lbs) morph into a mammal that weighs 25 to 40 tons - that's 50,000 to 80,000 lbs, kids. Would anyone believe that? I know it comes down to writing but it would take a lot to make that believable.

And how about going in the other direction. Could you make a 200lb muscled man shrink down to the size of a domestic cat (10-20 lbs) or even smaller - to a mouse weighing in at a groundbreaking 20 some grams.

JK Rowling made it work in her world with some of her animagus animals and creatures that had been transfigured. But would it work in urban fantasy/paranormal romance? Let's hear it folks. Weigh in on this weighty issue important to all writers of shapeshifters!