It will be very large.
Assuming that a standard batch of rice krispie squares is 30*20*5 cm in size and uses $5.00 worth of materials, a $1000 square could be almost 6 times bigger in every dimension, making it roughly 6 feet long by 4 feet wide by 1 foot tall.
Looks like the world's biggest is 12 feet long, 6 feet wide and 2 feet deep: http://www.iastate.edu/~nscentral/releases/03/jan/krispie.shtml. But I bet if you got the $1000, you could get Kellogg to throw in some more.
Also, you'd be buying in bulk, so that'd increase the amount the money went.
