Stimulus has to go on stuff that has lasting value greater than the value of the stimulus. That is the mistake that Gordon Brown made - you can't just slosh money around. The WW2 stimulus worked because when the dust settled, there was an industry expert at mass production, a trained and disciplined workforce, and a pent-up demand for exports.

In other words, what is at least as important as how much. You can't just blindly spend your way out.