Batch size is the batch of emails that should be processed in one block by the scheduled task. It is the number of emails that are read from the database, processed, and written back.
A lower batch size requires more database reads and writes and will hence be slower.
A higher batch size reduces the number of database reads/writes and, therefore, speeds things up. But it is important that the MySQL timeouts don’t get triggered, so it requires a longer MySQL timeout setting.