The Chlorophyta have both the smallest known eukaryotic cell and the largest known photosynthetic cells, with a smaller size range being found in the algal (charophycean) Streptophyta. Focusing on the smallest cells, predictions of how the limits on miniaturization of essential cell structures (e.g. the occurrence of mitosis with 20, albeit small, chromosomes with high gene density) are related to the size of the smallest green algae, suggest that the smallest green alga is close to the lower limit of cell size. The properties of the smallest green algae agree with predictions based on how physics, chemistry and biochemistry interact with small size for photosynthetic properties such as light harvesting and processing in thylakoids, but do not align so clearly for nutrient acquisition and assimilation. The energetic cost of volume regulation in flagellates in freshwater flagellates may help rationalize the absence from fresh waters of flagellates as small as the marine Micromonas. The minimum size of essential algal cell structures and prey size may help explain why there are no photosynthetic phagomixotrophs smaller than Micromonas.