The Block size determines how the data within a package is aligned in terms of bytes. Empty data is used to pad out entries in the package.
A block size of 1 can be used for most platforms. However, when using Wwise's Sample low-level I/O hooks, Windows and Xbox platforms require a block size that is a multiple of 4096, matching the size of sectors used for unbuffered I/O. Other implementations of the low-level I/O hook that your project is using may have different requirements.
This value will be specified as part of the File Packager project settings, or, if a project is not being used, as one of the arguments in the command line.
For more information about how block sizes are used in the runtime, refer to the Low-Level I/O section of the SDK documentation.