I have always used iron levels as an indicator, not a problem in itself (adding to a wear issue or causing other wear).

In a combustion engine (particularly diesel) is there any research that indicates that there is a point where iron in the oil contributes to wear?

If so, at what level?

I am hearing of moves to filter it, and that makes no sense if it does no damage, and you then loose your indicator because its being filters out/

Last question, how large is an iron particle?

Why would you think that iron particles in oil are harmless? I think the opposite, and would consider using any method in the book, including filtration, to remove it from oil. I would especially make sure to remove iron particles (and all others particles) at sizes close to the thickness of oil film(s) at the operating machine to prevent a three-body abrasion from happening. The order of sizes of iron in oil is almost unlimited. They could be as small as submicron colloids, and up to 1000, 2000 microns and even bigger. The size is dependant on the wear mechanism(s) that is/are taking place.
Also, there are other parameters, much more reliable than iron, to be used for monitoring and tracking serviceability of motor oil and/or machine.