MacBook – Operation failed when I tried to create a partition

The first thing I would try is to open Boot Camp Assistant and see if you can remove the partition. Whether Windows was installed or not, this should be possible.

Alternatively, since I'm guessing you haven't repartitioned your drive since you installed El Capitan, you won't be able to resize the partition unless you use a source outside of your computer. Even in recovery mode, you won't be able to repartition. You might be able to resize it in Disk Utility if you use Internet Recovery: CMD + Option + R at boot. Another option would be to boot into recovery mode, install OS X on a USB drive, then from the installation on that USB drive, you would for sure be able to resize your partition.

I have done this many times, so I understand the frustration it can cause when you have space that you can't recover and the only way to get it back involves several hours of your time.

The good news is that if you ever start with a fresh partition in El Capitan, their new partition scheme is resizable even when you're using it live so you wouldn't need to ever even touch Recovery Mode.

This link seems to explain it pretty well. Though HFS+ is well within the bounds of creating a partition that size, Disk Utility attempts to prevent you from creating any partition under 10% of the size of your disk. You'll want to use diskutil from the command line instead. Take a look at partitionDisk, resizeVolume, and splitPartition to determine which one is right for you.