You can find some information in these documents, not about bonding size, but the recommendations on why/when you need the bonding across flanges or in piping:
British Standard: BS5958-1 and BS5958-2
Basically, the recommendation is that the resistance to ground at any point of the system not be greater than 10x6 ohms to effectively dissipate static. So, if you can do that without bonding, great.
We bond our powder handling system piping due to some interpretation of the British Standards. In it is stated the same 10x6 ohms is adequate, but then it goes on to state that, in general, piping is usually somewhere tied to ground for electrical grounding at a resistance commonly lower than 10 ohms.
So, there is a section that states something to the effect of, "well, since electrical grounding is 10 ohms, we'll just make our recommendation at 10ohms..."
But there is a huge difference between electrical grounding and static electricity discharge/relaxation.
Unfortunately for us, our EHS dept took the 10 ohms. We spent over 7 digits (US$) to comply. There are 5000 bonds in our plant just for powder handling piping.
This is normally the space where people post something insightful.