I have not found a good answer in a search for the finite size range for solder microspheres. Glass microspheres have a defined size range of 100 nanometers to 5 millimeters in diameter. That’s too big of an upper range for solder. Forget 5mm – even .5mm is a large solder sphere. To set some size range, I think ≤200um diameter makes sense. Let me know what you think.