I saw an article yesterday that tried to break down the idea of a trillion dollars into something the reader could visualize.
The article pointed out that if you stacked a trillion $1 bills (US) on top of each other, you'd form a stack that reached a quarter of the way to the moon--the stack would be nearly 70,000 miles high. (Alternatively, if you formed a hundred stacks of $100 bills, each stack would be 7 miles high.) As the article noted, that distance is still kind of unimaginably big by our normal scale.
So here's another way of looking at it:
Imagine laying out dollar bills, on flat ground, edge-to-edge, so that they cover the ground. Wallpapering the ground with dollar bills.
A dollar bill's dimensions are about 2.6" x 6.1" x .0043"
If my calculations are correct, it would take about $253 million to cover a one-mile by one-mile square. So a quarter of a billion dollars is a square mile of money. (In $1 bills.)
So $700 billion in one-dollar bills would cover an area of about 2700 square miles, or a 52-mile-on-a-side square. That doesn't sound like a lot to me in those terms. But think about it: Draw a line 50 miles long. Now draw three more 50-mile-long lines to turn it into a square 50 miles on a side. If you're familiar with the eastern US, you could take the state of Connecticut and draw a line down the middle of it; each half is about 2700 square miles. Now cover every single inch of the eastern half of Connecticut with dollar bills, and you've got $700 billion.
If that still doesn't sound like a lot of space to cover, note that it would take about 20 hours to walk along one edge of the square. Or think about driving, at freeway speeds, for 45 minutes, with nothing but a field of dollar bills stretching out further than the eye can see in all directions.
Okay, one more try: $700B in one-dollar bills (if packed perfectly) would fill a volume of about 28 million cubic feet, which is roughly half the interior volume of the Metrodome, the Minneapolis sports stadium. Fill a football stadium halfway to the brim with dollar bills, and there's your $700B. (I suspect that if you just tossed them in, rather than packing them perfectly, you'd fill the whole stadium. Though I imagine they would also compress somewhat with weight.)
Note: This entry is purely an exercise in visualization; it should not be construed as making any kind of political argument about whether it's a good idea or a bad idea to spend $700B. Also, this entry ignores all sorts of real-world constraints, like weight, and land not being completely level, and so on.