I'm watching the same tiny 1-wagon train make trip after trip between the same two stations (the supplies and the request are for 10 wagons total), while 2-wagon and 3-wagon trains are idle at a depot. Is it possible that (possibly due to the "skip depot" functionality) the trains aren't reviewed between deliveries, so whichever goes on the first delivery keeps going on following ones, even if a larger/smaller train would be more efficient? Both stations are limited to 1 train.

It is choosing the same train over and over because it is closer due to the fact it's skipping going back to the depot. I'm not really sure there is a good way to fix that on the mod's side, you'll just have to use larger request thresholds.

Would it be possible to have it as a mod setting? Something like "Prioritize: [Distance] or [Delivery amount]"?

I have built up my rolling stock to have equal amounts of each size of the train, to take advantage of the fact that an automated train control system can match them to the amounts needed. 4-wagon trains delivering 1-wagon's worth of materials (and vice versa) isn't just inefficient on its own, in a busy enough network it can also slow down other deliveries by unnecessarily clogging up stations and routes. I would love to be able to tell the mod to choose capacity first, and only then consider distance.

Yeah I can implement that as a setting.

I decided to just make prioritized cargo capacity the default behavior in 1.2.2