Obviously too much spare time.
So I took a brand new stack of $100 (actually $2) bills which measured 0.5 in by 6.25 in by 2.625 in or 8.2 cubic inches. Dividing by 1728 equals 0.00474537 cubic feet per stack. Multiply this by the 2,200,000 stacks of $100 bills ($10,000 per stack) that would constitute $22 billion and you get 10,440 cubic feet. So $22 billion would be (coincidently) a cube approximately 22 feet on each side.
Now if the $22 billion were well used $20 bills then......