The CoreXY mechanism requires at least 8 rollers for guiding the belts. And so the set of 10 ball-bearings (set of 10 for MakerBeam) that I had seemed a good candidate for making the rollers. These bearings are 5 mm thick, have an outer diameter of 13 mm and 3 mm hole.
And it worked out fine. Had to fine tune the model a few times for a tight fit, but in the end rollers looked great and rolled very smooth.