How to calculate RAM disk size on OS X Mojave—500 or 512 byte block size?
This seems simple enough. You asked for 12000000 sectors. The sector size is 512 bytes. So 12000000 * 512 = 6.1 GB which is the size of disk15. If a block size of 500 bytes was substituted for the 512 byte sector size, then the calculation would be 12000000 * 500 = 6.0 GB. This is not what is shown.

The output from diskutil info disk15 | grep -e Size should be as shown below.

   Disk Size:                 6.1 GB (6144000000 Bytes) (exactly 12000000 512-Byte-Units)
   Device Block Size:         512 Bytes


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