Just for fun, I was looking through some sizing specifications for crushed rock ballast, from my mining days. Here are a few typical sizes of ballast produced by various crushing companies. The size fits into a very tight size range as you can see.
Maximum size x Minimum size
2 1/2" x 3/4"
2" x 1/2"
1 3/4" x 1/2"
1 1/2" x 3/4"
Of course, the best ballast for your model railroad is always what looks best to YOU!
Jeff