Just check Split Files and enter the size you want. If the file is set to 15.000.000 bytes and the result is 14976000 bytes it's just because 1 KB =/ 1000 bytes. The size is calculated at 1 KB = 1024 bytes. This is not a bug, it's just how the space is calculated in computer storage.
The files are split based on the configuration options (blocks, cluster size, etc) which can be edited from the configuration window.