I am thinking about not reinstalling it. I am trying to get my OSX partition to be as small as possible. Currently its around 25GB. What can I do to reduce this?
As mentioned before, you can choose not to install printer drivers and additional languages.
And if you DON'T install iLife, you will save around 4 to 5 GB too.
Thus the numbers Intell and me gave you should be reachable, but know, that you should factor in space for temporary files and for swap space.
Why do you need to install a bare Mac OS X 10.5 Leopard?
Well, I will get shot on this forum for saying this but this MBP will be my Win7/Win8 laptop now.