Here’s a histogram of pollen sizes (~30 grains per species) from 3 individual plants of Coreopsis palmata, Echinacea angustifolia, and Heliopsis helianthoides.
Greg outlined the methods taking the measurements here. Greg, what software program did you use?
For the measuring of the pollen I used the scopephoto software that came with the camera. It had a setting for pixels since the magnification and field of view were consistent for all images. I tallied it with an excel sheet.