Answer by Brian for Where is the response size in Squid's access.log?

The native log format is:

"%9d.%03d %6d %s %s/%03d %d %s %s %s %s%s/%s %s"

With the 5th part (%d after %s/%03d) being:

bytes The size is the amount of data delivered to the client. Mind that this does not constitute the net object size, as headers are also counted. Also, failed requests may deliver an error page, the size of which is also logged here.

ref: http://wiki.squid-cache.org/Features/LogFormat