The Satellite is a KH-11 type imaging satellite, or based around that model. It weighs appx, 5,000-10,000 pounds depending on how you're talking too (for many reasons) and its about the size of a schoolbus (depending on who you're speaking too of course).
Now, what is so dangerous about this satellite it has to be shot down you ask? Why is it so bad- well its bad for 1 very public reason (which is still bad) and its bad for a not so public reason (which is dam near scary) that is easily inferred.
The Satellite contains Hydrazine- yes. It is likely that none of it has been used on the Satellite, yes yes yes. When the satellite comes down, its orbital velocity and its makeup suggest large parts of it will survive the burn up phase, and more worringly, the Hydrazine will be dispersed in an aerosol like fashion over a small area, rather than a large one. This is possible- if it happens hundreds of people could get sick, and a few would likely die over populated areas. If people were to come upon the wreckage, that too could make you ill and kill you.
Now, for the not so public part. The Satellite may be powered by an RTG (radioisotope thermoelectric generator) which means it would likely also have a sizeable quantity of Plutonium on board, or thorium, and something new. These radioactive fuel rods power the spacecraft in space for long periods of time when solar isn't sufficient, or power demands onboard the spacecraft are higher than solar can allow. This fuel is the real problem. While, its probably a smaller amount (less than 100 pounds) it has the ability to poison a whole region if it comes down in the wrong way. Just a few molecules of this fuel will kill you after 3-5 years of exposure (if you breathe it in, drink it, eat it, etc). This fuel has the potential to kill hundreds of thousands of people over a period of several years if it comes down near a populated area. Thus the fear on the part of military top-brass.
I am only telling you this so you know the danger. During the shutdown, if you are anywhere near the target I suggest you leave, immediately. If you see debris, you must treat it like it is radiological and evacuate the area, towel over face. Do not eat anything, or drink anything in the vicinity of where you see something either burning to the ground, or on the ground. You are asked to leave because it will literally save your life. Keep your ears open too, anything close to you, debris trails above your state, debris reports on the ground, you are asked to leave that area, and get as far away as possible.
If you're in California, I suggest you go to Florida. If it comes down in Florida, I suggest you go to California. You get the picture. This thing is extremely dangerous and you are asked, and warned to stay away from it, for your safety.