35mm, 24x36 frame size - 920mm2
IIRC, eliminating the waste area of the sprocket holes was one of the selling points of 828. Also IIRC, the film and backing from 126 cartridges could be spooled onto 828 rolls. I last saw 828 for sale in the mid 1970s. The art-decoish Bantam 35 RF was one of the all-time coolest cameras ever made IMO.