I wrote a 30,000 document on Black snakes and during research i found one reliable record of a 2.54m individual. I would catch more red bellies than just about anyone in sydney and i have measured them all and the biggest has been about 1.6 m,but one of our customers has showed me a picture of a hefty one that lives next to an RSL club i would like to catch and measure it has quite a girth.
I have no doubt that they can reach 2m,but funny thing i have had 2 brown snakes this season that exceed 1.8 m in length but are not as robust as bigger red bellies.