Switchers from non-UNIX-based users are baffled why there is a need for a swap partition and/or file. Little did they know that most modern OS have their own way of doing this and it is just that in Linux, you are free to tell the OS where and what size you want your swap to be.
Determining the swap partition size is not rocket science. Personally, my rule is for RAM of less than 1GB, allocate a swap space that is double your physical ram. If you have 1GB or more, allocate the same amount as your physical space. Again, there is no set rule in doing this.
To give you more information about Linux swap space, check out "All about Linux Swap Space."