Name the largest unicellular algae

Unicellular algae are plant-like autotrophs and contain chlorophyll. They include groups that have both multicellular and unicellular species:

Euglenophyta, flagellated, mostly unicellular algae that occur often in freshwater. In contrast to most other algae, they lack cell walls and can be mixotrophic (both autotrophic and heterotrophic). An example is Euglena gracilis.
Chlorophyta (green algae), mostly unicellular algae found in fresh water. The chlorophyta are of particular importance because they are believed to be most closely related to the evolution of land plants.
Diatoms, unicellular algae that have siliceous cell walls. They are the most abundant form of algae in the ocean, although they can be found in fresh water as well. They account for about 40% of the world's primary marine production, and produce about 25% of the world's oxygen. Diatoms are very diverse, and comprise about 100,000 species.
Dinoflagellates, unicellular flagellated algae, with some that are armored with cellulose.[48] Dinoflagellates can be mixotrophic, and are the algae responsible for red tide. Some dinoflagellates, like Pyrocystis fusiformis, are capable of bioluminescence.