Size/grade of HO ballast?

[fire5506]

You might experiment on some hidden track to see what looks good. CN locally seems to have a mixture -- bigger chunks in the middle and fine at the edges but that was in a station.
I think that it may be laid as large chunks and get broken down over the years. I think ballast is one of those features that need to look right rather than be scale.

The bigger stuff is to hold the track and the finer is for walking on. The finer ballast at the edge of the tracks is called "walking Ballast". It is to make it easier and safer for the crew members walking alongside the cars.