paging file, as in your virtual memory? Find out how much memory your programs need at most, and set it to be equal to that amount minus your how much RAM you have. Depending on what OS you use there are a number of tools to track memory usage.
Overly large pagefiles really don't do any harm on NT OSes, but they don't help either. Aside from putting it on another partition like Tebore said, or, much preferred a totally separate drive on a different IDE channel, there's not much optimizing to be done
I find that the best config for the page file is to make it waht ever sixe you are comfortable with (not to big) and spread it across multiple Physical Hard Drives. Do not stick multiple page fiel if you only have one hdd, becasue the other partition is logical you will cause your hdd to go crazy. Only to it with two or more.