Original: Many species are single-celled and microscopic (including phytoplankton and other microalgae); many others are multicellular to one degree or another, some of these growing to large size (for example, seaweeds such as kelp and Sargassum).
Simplified: Many types have one cell and are very small or microscopic (including plankton and other algae); many others are have many cells, some of these being very big, like seaweeds such as kelp and Sargassum.