Michael,
Batch size does not mean that system will send specified amount of emails per scheduled task run, it means that system takes specified amount of emails (150 in your case) from queue, processes them, takes another 150, until there are some emails in queue.
Sending emails is quite simple task, so I'd recommend you to implement your custom scheduled task and disable default one, as I'm not sure there is a setting that will allow you to specify the amount of emails to be sent out per task run.
Thanks,
Roman