We investigated regional diatom species distribution, local diversity, and spatial variation in body size from 105 boreal stream sites in Finland. Stream diatom body size distributions ranged from a slightly right-skewed pattern at the largest study extent, comprising the whole country, to symmetrical distributions within watersheds. Compared to earlier findings for larger organisms, diatom body size distributions were more symmetrical, implying either narrower study extent or fundamental differences between unicellular and multicellular organisms. There was a weak positive correlation (r = 0.239; P < 0.05) between latitude and average maximum cell size in stream diatom assemblages. Multiple linear regression showed that average maximum cell size was best accounted for by a model incorporating moss cover and latitude. There was also a weak (r = 0.290; P < 0.01; n = 105) negative correlation between local species richness and average cell size corroborating the earlier findings for other organism groups.