The existing answer didn’t really answer the question.

It has to be noted that much of useful advice on tuning resides not in the docs, but in forum posting by nginx developers.

On the OP’s question, Maxim Dounin gave excellent guidance as early as 2009

Some hints about seveal big vs. many small buffers:

• Using bigger buffers may help a bit with CPU usage, but usually it's not a big deal.
• Using bigger buffers means less disk seeks if reply won't fit into memory and will be buffered to disk.
• On the other hand, small buffers allow better granularity (and less memory wasted for last partially filled buffer).

Since there are different load patterns (and different hardware) it's hard to recommend some particular setting, but the about hints should allow you to tune it for your system appropriately.