Definitely don't generate keys using an online service. Maybe it does the right thing and never sends the keys to the server where it's hosted, but that kind of behavior might change over time, i.e. the server might get hacked, etc. Instead, the CLI wallet can be used to generate the keys locally.
So even if you put one block_log, it will split it first and you should pay attention to your storage space in this case as 1TB might not be enough.
Hmm, does this mean that when block_log is split into parts, it takes up much more space?
It doesn't consume the original block_log when generating the parts. So you end up with 2 full block_logs. Hence more space usage while splitting.