Ataman Does anyone know the cross size...

As Scandinavian I have used metric system my whole life and I also have been working on a garage for almost a decade.

O-rings are categorized by inside diameter x thickness of the rubber, which in this case would most likely be 3,5x1,5.
I believe the cross section means thickness.
Otherwise you would have a funny looking O-ring with a miniscule 1,5mm hole and a 3,5mm thick rubber around it.

And yes, they are firstly sold by inside diameter. So just to make sure, the overall diameter of this O-ring would be 3,5mm + 1,5mm + 1,5mm = 6,5mm but no one uses that measurement. This was just for reference.

For what it's worth, I understand very little about inches and feet.