The minimum value that the user can code is 32760. A reason that someone might want to limit block size to less than the device maximum is that the user might want to copy the tape to another device type without reblocking. This requires the user to know the maximum block size supported by the device. IBM recommends letting OPEN calculate a block size if feasible. You might want to choose for your program to take advantage of the system-level default for copying programs.
All trademarks are properties of their respective owners.