I don't know, Dave Willoughby taped various WSM contestants for an article in the May 1983 Ironman.
He gave the following measurements for:
Bill Kazmaier:
Biceps:21.4
Forearm:16.6
Weight:320
Height:6' 2"
Wrist: 8.6
Ken Patera:
Biceps:21.7
Forearm:16.8
Wrist:9.5"
Ht.6' 1" .5
Weight:345lbs.
Those were measurements taken for two high level strongmen, each about the same height as Arnold, but 80-100lbs. heavier than Arnold at his competitive weight. look at the arm measurements. Now, granted, they'll be some variation do to training style, etc.... but it wouldn't be that large. I'd guess his arms were 19ish in that photo. He might of taped out larger when he was a offseason 260lbs, but not 3 inches more.