I have the metering back 8x10 Sinar F1. Looks like it was cut to english measurements. The 8x10 glass is exactly 8.875" or 225mm by exactly 10.5" or approximately 266mm. Using the 25.4mm/inch conversion factor this would be 225.425mm x 266.7mm. This is original Sinar glass in a metering back. It has rather large clipped corners. They're 0.875" or 22.225mm on a side. The diagonal of the clipped corner is 1.25" or 31.75mm. The glass thickness is 0.073" measured with a micrometer. (Basically 1/16" glass).
The non-metering back is smaller. I don't have those dimensions.