This is a relatively uncommon sensor size mid-way between APS-C and full frame. Canon used it for its EOS-1D high-speed pro sports/press photography DSLRs before these were merged with the introduction of the full frame EOS-1D X.
APS-H sensors measure approximately 30 x 20mm, or a couple of millimetres less.
Canon has since announced the development of a 250Mp APS-H format sensor, though this has not yet been used in any commercial product.
Sigma, meanwhile, has announced a new Sigma SD Quattro H mirrorless camera with a new APS-H format Foveon sensor.