iF you have already created the Windows partition, you can't resize it. To do it, you have to use an app like Paragon Camptune, that will allow you to resize your Windows volume > http://www.paragon-software.com/downloads/camptune.html
The other option is to delete Windows and create the new partition with the size you want in Windows. Make a backup of your data before doing this
I tried Paragon Camptune X and had no success.
However, another Paragon product, the Paragon Hard Disk Manager Suite, did solve my problem. Installed it on the Windows Boot Camp partition, opened the app, selected Partitioning/Resize Partition, and used the slider to reallocate hard drive space between Mac and Windows. The software runs, reboots the system to a BIOS-like screen while it reallocates the space. It takes a few minutes but it worked pretty flawlessly.