I think overall your most common stock/rail/rod/conduit size worldwide is going to be 1″ or 25.4mm. Second is going to be 25mm. You could easily account for this by using some sort of eccentric spacer with the bearing holes. This would also be good for adjusting pre-load/tension on the rails. I mean you can buy eccentric spacers, get them made in bulk for cheap from china, make them yourself…drill a piece of round stock off center or and even print them. If you aren’t sure what I am talking about, it’s basically a nut where the hole is off center, that way when you tight it it moves the bearing/wheel closer or further since it’s off center.