Use the grid tool...press G and you can adjust how big each square (in pixels) is..
i agree.
but i also agree with Meganuke, it would be usefull for knowing the exact scale of a city to make a really realistic bomb!
What I think he/she/it meant was that there would be different effects of the different scales on the elements/air pressure. for example, if say in far scale, pressure would build up and disperse quickly, with the pressure not spreading much, and flames being quite small. however, in the close up scale, pressure would spread rapidly, and flames would be bigger, etc. just basically recoding the elements/air pressure a couple times.