Actually, I believe the more commits you do, the more your performance will suffer.
Each commit forces the log writer to write so when log writer writes more often, it does more job and gets slower.
Rollback segments is what you should be concerned about with the large transactions and if they will be able to handle the transaction
If I was you, I would test with different transaction sizes, using small parts of the data, so that I know how really my system performs better, and then I would go for deleting the real part of the data.
I believe that definitely should be done off hours

Regards