I spent most of my morning cataloging the different size bars the different trim levels use
Unfortunately I can't find the diameters of any of the front bars anywhere despite searching for hours.
On the rear the bars are anywhere from 18mm to 25.7mm stock G30 (non M5) and the M5 non competition has 32mm front and 30mm rear.
With that said, we can't really know how stiff any of the stock bars are as we don't know whether they are solid or hollow and if hole what the wall thickness is...
The best way to upgrade I guess is to buy them as pairs for aftermarket (so both front and rear same brand & "stage") or to upgrade to OEM parts as a set with maybe the rear bars one "step" thicker.
Doc here
https://docs.google.com/spreadsheets...u_N1KQ/pubhtml
Depending on market, model, trim, option package you could have anything between 18mm to 25.7mm rear ARB in a non Mcar 5er sedan, the 5er wagons run stiffer rear bars like for like compared to the sedan.