Support are the log analysts, this kind of how-to question makes for a good severity 4 case, though if a job is failing that's at least severity 3.
However, I happen to know the answer in this case:
In a Job.*.log file, search for "<StgBlockSize>". For example, in a backup copy job, this should show "Automatic" because this is the setting, not the actual size used.
In a Agent.*.log file for a data mover that works with a backup file, search for "block size". This is what's actually used.