Hi,
you will need the block size of 64K only when doing disaster recovery, in all other cases I recommend to use a higher value (256 or 512). Why it a smaller block size needed for Windows in case of using EADR? Windows cannot handle a block size > 64K when the drivers for the tape device are not loaded. This is the case when doing EADR and the Windows is running in safe mode or in PE mode.
So I recommend to have additional logical devices defined for your library (i.e. Lib01_Drive01_64 in parallel to your exisiting device Lib01_Drive01) and set the block size to 64K. You will need to do the backups for the cell server (if Windows) to the 64K devices and for the other Media Agents. All other clients can be backed up using the higher block size.
Best regards
Daniel