While the documentation on wal_buffers suggests that the default of 64 KB is sufficient as long as no single transaction exceeds that value, in practice, write-heavy benchmarks see optimal performance at higher values than you might expect from that, at least 1 MB or more. With the only downside being the increased use of shared memory, and as there's no case where more than a single WAL segment could need to be buffered, given modern server memory sizes the normal thing to do nowadays is to just set:
wal_buffers=16MB
Then, forget about it as a potential bottleneck or item to tune further. Only if you're tight on memory should you consider a smaller setting; a good starting point is shown at http://pgtune.leopard.in.ua/.
