El Capitan - Increasing/Resizing the BootCamp Partition Size
The product I have used for years to solve such a problem is Winclone. According to Russel Scheil at Twocanoes Software, the Basic edition is all you should need. You install Winclone as an OS X application. The steps to create a larger Boot Camp partition are given below.


Use Winclone to create a compressed backup up of your Boot Camp
partition. This backup can be stored in any Mac OS Extended (HFS+)
formatted volume.
Delete your Boot Camp partition.
Resize your OS X volume.
Create a MSDOS (FAT) formatted partition. This partition will need to the same size or larger than the original Boot Camp partition.
Use Winclone to restore Windows to the MSDOS (FAT) formatted
partition. Winclone will change the partition to NTFS format. If
Winclone is running under El Capitan, you will first need to disable
System Integrity Protection (SIP) before restoring Windows.