The small round is about 3/16 inch. The large round is maybe 5/16 or 3/8? It’s bigger than a quarter inch, for sure. But I kind of mash them around making them more oval than round, so once I’ve used it, it’s hard to be sure what the original size was.
I also sharpen the back side first time I have to sharpen the front, so then I’ve got a cutter that lasts twice as long. ;-) The cutter is just a thin strip of O-1 that’s been bent to shape, I think. It’s decent tool steel, but doesn’t last forever.