Centering an image

Hi, I’m trying to print some stuff with a PS backend
(python 2.4 with WinXP). I have a figure, then I save it with savefig. Right
now the figure is centered in the page. However, if I change the page size
(such as when viewing with ghostscript), the figure is no longer centered. In
fact, I’d like to control its x and y offset because it’s a really
large image and I only have letter or 11x17 size paper, so I’d like to
print it out in panels. How can I do this? Also, how can I specify 11x17
paper in savefig?