I've never questioned the size of 0.4mm nozzles. To me the ones I have look right but its very hard to judge.
An undersize hole is not critical, unless it limits the flow due to back pressure (it would need to be quite a bit undersize). Oversize (as I've found to my cost) is more problematic. What appears to happen is that it amkes teh delivery inconsistent and you counter-intuitively get symptoms of under-extrusion.
I find that with extreme wear the bore opens out and I get inconsistent results (I suppose I could try setting Cura for 0.5mm, but as nozzles are just pence…)
I've had a play with 0.1mm nozzles. Life becomes very slow and things like bed adhesion become very fussy.
This 100 tooth wheel took a long time to print, the top surface is really good. In the end I used one printed with a 0.4mm nozzle which isn't as pretty but works well. For the matching 20-tooth wheel the 0.1mm nozzle gave a much better result. I think 0.1mm nozzle with 0.5 or 0.25mm layers would give results to match resin printing, but very, very slow.