Many people have already brought up the many, many reasons an actual giantess cannot exist, but… if my blog is to be really complete, I think that means I have to touch on this as well.
Fortunately, this means all the really good research has already been done and all I have to do is summarize it here. (Why’d I choose a 100′-tall woman? To her, a man my size would be 4″ tall, and that’s my ideal height with a giantess.)
The first thing most people bring up is the “square/cube law”. This means that the volume of a 3-D object increases much more rapidly than its surface area as you enlarge it. People use the cube for a model: if you double it in all dimensions, it becomes twice as long, wide, and high, but its volume is now eight times what it was. So you can grow a 5’6″ woman to 100′ tall in appearances, but she shoots up from weighing 100 lbs. to just over 300 tons.
The Straight Dope also indicates the problem with the circulatory system, how much blood now has to be pumped through greater distances, simply to carry nutrients and oxygen at a similar rate. So maybe a giantess could have several hearts distributed throughout her body, but she still needs exponentially larger, denser bones to support all that flesh and muscle, not to mention her mass is producing much more heat than her surface area can dissipate. Even reaching out to swat a tiny explorer, Straight Dope suggests, would take enough energy and produce enough heat to require her to recuperate for hours in a pool.
Not only would a real giantess be confined to large bodies of water, like a whale or the Loch Ness Monster, but she would be grotesquely disproportional in contrast with our expectations of a huge, sexy goddess striding around imperiously.
If she could hold herself upright, every footstep of a 100′-tall woman weighing 300 tons would release 1.7 million joules of force, triggering just under a magnitude 1.5 earthquake on the surface of the land. If she ran, she’d be traveling at 175 mph, every stride reaching as long as she is tall, producing nearly magnitude 2 earthquakes as she went. You might notice that if you lived in the area: pictures and mirrors don’t fall off the wall until a magnitude 2.5 earthquake.
So there, that’s out of the way.