A lot of people tend to miss the basic idea behind the swapfile. It's supposed to back up the RAM, not replace or add to it. In other words, it's used only if you don't have enough RAM. Therefore, the more RAM you have, the smaller your swapfile needs to be.
How small (or large)? That depends on two things: 1) How much RAM do you have? and 2) How much RAM do you use?
To answer both of these two questions in one step: Set Windows to Manage the swapfile, open Microsoft's System Monitor (see next paragraph), set it up to monitor the "Swapfile In Use", and then push your PC system to the limit. Open up as many programs as you might ever use, run as much as would be normal for you. This will create the largest swapfile you will ever see on your machine.
If you don't see System Monitor, look for C:\WINDOWS\SYSMON.EXE, or you may have to add it from System Tools using Windows Setup in Add/Remove Programs.
Once you know the largest swapfile size for your PC (for example, 30MB), then add a safety margin (for example, 30 + 10 = 40MB). Now fix the size of the swapfile to the max+safety (for example, 40MB).
It's also better to defragment your hard disk before you fix the swapfile size. This will make the swapfile contiguous (all in one spot). Even better is to place the swapfile on the outer edge of the platter in the hard disk using a program like Symantec's Norton Speed Disk. This will help improve efficiency by placing the swapfile in the spot where access is the fastest.
This procedure will optimize the swapfile for your particular system.