Big, modern ballast is in the 3-4" range, earlier ballast is smaller, cinders are in the 1/2 to 1" range.
For O-scale, 3" ballast would about 1/16" in size, 1" ballast would be in the .020 inch range, roughly 1/64".
As was said before, O scale ballast or stuff about 1/16" of an inch should be...