Honey, I Shrunk The Kids is incredibly unrealistic. I mean, aside from the REALLY OBVIOUS ones. Shut up. :-p

The point here is that they are being shrunk really small. Do you have any idea how much energy it would take to shrink someone that small? We all know that pressure and temperature are related, and realistically you couldn't just jump to such a small state without passing through every possible state in between; in other words, you are being compressed. Pressure is directly proportional to temperature according to the combined gas law (Gay-Lussac), you know, so if someone were shrunk that small, the heat would probably be enough to kill them. I don't want to do all that math right now. (note to self: do this later!!)

Also, I'm pretty sure that a lot of forces that humans take for granted because they are so small - the energy from radio waves, some kinds of light, maybe even things like magnetism and background radiation, hell, there are all kinds, just think for two seconds - would have a much greater impact on the human body if it were that small. Also, simply based on the observation that other animals that small have heart rates that are WAY FAST, human hearts are not designed to beat so fast, so you'd have tachycardia and, eventually, cardiac arrest or heart failure.

You'd probably go into shock also (at some point before you start to die) and lose consciousness, either from the sheer speed of the physical changes you are undergoing or from the pain of the heat and your heart and all that damn energy. Also, where would you get all that power from? I'd imagine it takes a lot to compress a human being. You would also weigh exactly the same amount unless that ray somehow removed matter from your body, which would also probably hurt. Also, it would cause infinite genetic mutations if it worked on an atomic/molecular level because it would remove a different pattern of information from every single chromosome in your body. You would probably just lose all essential function in your whole damn body, and if the shrinking mechanism worked by just making the atoms smaller, it would cause the laws that govern the universe on an atomic level to function differently, with unpredictable results. If that were the case you wouldn't be able to breathe anymore because all the oxygen molecules would be "huge" to you and your cells wouldn't know what to do with them, if the hemoglobin even managed to get them there.

Reference to muscles and their strength proportional to the rest of you, vague comparison to a flea and a football stadium... I will finish this some other time.