Following can help you tweak fastcgi_buffers size & numbers.
Maximum Response Size
awk '($9 ~ /200/)' access.log | awk '{print $10}' | sort -nr | head -n 1
Please note we taking HTTP 200 OK response only into consideration.
Average Response Size
echo $(( `awk '($9 ~ /200/)' access.log | awk '{print $10}' | awk '{s+=$1} END {print s}'` / `awk '($9 ~ /200/)' access.log | wc -l` ))
Based on above result, for rtCamp.com we found following values:
Avg. 24807
Max. 629622
So we are using:
fastcgi_buffers 32 32k;
fastcgi_buffer_size 32k;
Related: Nginx Log Parsing Examples