wal_buffers

Can anyone tell me what precisely a WAL buffer contains,
so that I can compute an appropriate setting for
wal_buffers (in 8.0.3)?

I know the documentation suggests there is little
evidence that supports increasing wal_buffers, but we
are inserting a large amount of data that, I believe,
easily exceeds the default 64K in a single transaction.
We are also very sensitive to write latency.

As background, we are doing a sustained insert of 2.2
billion rows in 1.3 million transactions per day.  Thats
about 1700 rows per transaction, at (roughly) 50 bytes
per row.