Hi Ross,
I was thinking about the precision ground round rails, 20mm for the longest axis - probably for the others as well, though I may go down to 16mm for the Z. My guess (if I am wrong please tell me) is that unsupported is the best way to go if short rails are used (my longest rails will be 400mm) while the open bearings for supported rails would give more slack but reduced spring in the rails themselves.

Mike