I've been experimenting with the concept of using subtended angles on the retina. For devices that can sense the distance to the user, we'd use that, otherwise

I've been experimenting with the concept of using subtended angles on the retina. For devices that can sense the distance to the user, we'd use that, otherwise you make an assumption based on the kind of device. That way you should be able to get sizes that appear roughly the same size regardless of whether they are on a huge screen far away or a small screen up close.