I think a better way to put my initial question would be: what is that pixel size unit issue, what's it about and what problems will be gone with the fix?
Actually, I would call that 'choice', more than 'issue'. Rendering a lot of thin, overlapping objects, that's always a challenge. Regardless of world or screen space, you'll have to 'do something' with, for example, thin grass which is far away from camera, that constantly 'buzzing' from frame to frame. At this stage, current, screen space, seems to be easier to control.
It's not big deal to create compound that changes strand size according to output resolution, distance to camera and camera angle, but this one can kill the playback. Also you still won't get it right in render region, unless you manually enter a some 'estimate' values.
Anyway, I'll try to create compound.