A laser diffractometer and three image-based instruments with spatial resolutions between 0.33 and 10 µm/pixel were compared through measurements on calibration spheres and fine fractions comprising pulp fines of various types, neat PCC filler, and a mixture of fines and fillers. The laser diffractometer was highly sensitive to the keyed in refractive index of the samples, which was calculated based on volume-based mixing rules. A high-resolution flow cytometer and a high-resolution fibre analyser were found to be complimentary for measurements on neat fines and fines/filler mixtures, and superior to the laser diffractometer. When measuring on fillers, the laser diffractometer performed as well as the high-resolution flow cytometer, which was capable of resolving single filler particles. The sizes of the calibration spheres were overestimated by the image-based instruments, and the measurement uncertainty was high. The uncertainty was mainly attributed to the unrestricted particle motion, and the low accuracy to the dissimilar optical properties of the calibration material, compared to fines. Thus, calibration materials with shape and optical properties more similar to fines should be developed.