I think you’re extremely unlikely to find smaller than 3-3/4 x 7 because that’s the size bearings West Side Lumber Co. used on narrow gauge log cars and other equipment (with some 4-1/4 x 8 on the larger cars). Even the smallest standard gauge rolling stock would have needed at least 3-3/4 x 7.

David Soderblom
Baltimore MD USA
drs@stsci.edu, 410-338-4543