Look for a program called Gparted in Linux. You can find it in the Software Center. It allows you to change Mint's partition size to free up room for a Windows partition.
However when you install Windows it will replace Linux's bootloader. So Linux won't start anymore. That needs to be fixed.
The best way to make a Windows / Linux dualboot system is to install Windows first, then Linux. Because the Linux bootloader will automatically detect Windows and add an option to it to start Windows. Windows bootloader only sees itself.
You might want to consider doing that. Especially since Mint 16 is nearing it's end of life in July. Mint 17 will be out soon. That one will have five years of full support
I'm running the RC version of it. Basically the finished product, but the Mint team is still fixing bugs in it.