As of when I quit last January:
That's the biggest they'll do in 1-hr. You can send it out, it'll take 2 weeks. Prices are listed on the kiosk when you order, or the sign on their counter. Some stores print larger in store using large format inkjet printers - prices I'm unsure of since we didn't have that in the store I managed. As stated before, if it's a professional image, marked or not, they won't print it without a release.
I think 7 to 10, maybe more if they charge you with terrorism. And, yes you can buy explosives at Walmart - black powder in the sporting goods section.
Well, generally explosives can not be bought at walmart. Since it is a 2d picture, you should be able to use a 2d dynamite stick such as http://www.creativecrash.com/marketplace/3d-models...
The cost would be 5 dollars, but you might want to get a second totaling 10 dollars
Depends on how big you want to make it. They are pretty cheap though. Make sure that it isn't a professional picture though. If it is they wont blow it up for you.
Gee... why don't you do what any sensible person does and ask THEM !!!