That's more than large enough for a briareus! You could get by with a 75 or a 90 gallon tank.
Let's compare: the sizes listed in Cephalopods: A World Guide are these
briareus body 5" arms up to 24"
bimac body 5" arms up to 14"
I guess one thing we have to remember with these octo sizes is that an individual may or may not reach maximum size. My bimac was a big one, arms maybe 22 inches - and some do grow large, but others don't. The adult briareus that I've seen have long legs, but the body doen't appear to be as large as that of many bimacs. They are beautiful octos.