Nobody working for Obama seems to know how big his public works program will be, but the usual number you hear is around $700 billion.
You know where they got that number: from Hank Paulson's original bank bailout. You can imagine the rhetoric already: "If we can afford $700 billion for Wall Street, we can afford $700 billion to fill the potholes on Main Street!"
And how did Paulson come up with his $700 billion number? As far as I can tell, he basically had no clue. He just wanted a really big number "to restore confidence," but he probably didn't want one so big that you could round it up to "a trillion." Hence, $700 billion, our new national lucky number.
Update: A commenter points out that when Paulson left Goldman Sachs, he cashed in for $700 million, which might be how he came up with 700 as his favorite number.