How much does the SD card hold? 2GB? 4GB? How much RAM do you have? The swap file size is directly proportional to the amount of RAM you have.
If you're looking to free space, I suggest you use a program like CCleaner (download from www.piriform.com ) to delete unnecessary files that clutter your system. If you don't use the Windows "Hibernate" function, disable it and delete the hiberfil.sys file. This will free up the same amount of space as your current system RAM. You can also trim down on programs and uninstall the ones you don't use often.
I think it's risky to keep the swap file in an SD card. If it accidentally comes lose, or if someone takes it off, you'll have a BSOD.
You might want evaluate your actual memory requirements to see if you can reduce the size of the swap file. The default size created by the system is usually a lot more than you really need.
You can do this by running Task Manager (Ctrl + Shift + Esc) and select the "Performance" tab to see how much memory you're really using. If the number below the memory bar graph on the left side of the window never gets above, say, 4GB and if you have 2GB of actual RAM installed, then you really only need 2GB of swap file, although you should allow some extra margin for safety.
Just make sure that you base your needs on the the number shown by Task Manager when you have the maximum number of open windows and documents that you ever use at any one time.