How to partition 128 MB space on El Capitan?
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.