Unicellular organisms are known to exert tight control over their cell size. In the case of diatoms, abundant eukaryotic microalgae, the layout of the rigid silica cell wall imposes geometrical restrictions on cell size. A generally accepted theory states that the need to fit any new silica element into a previously formed structure causes a reduction in size with each vegetative division cycle, until cell size restoration is achieved by a switch to another life-cycle stage. Nevertheless, several reported exceptions cast doubt on the generality of this theory. Here, we monitored clonal cultures of the diatom Stephanopyxis turris for up to two years, recording the sizes of thousands of cells, in order to follow the distribution of cell sizes in the population. Our results show that all S. turris cultures above a certain size threshold undergo a gradual size reduction, in accordance with the postulated geometrical driving force. However, once the cell size reaches a lower threshold, a constant size range is maintained by different cellular strategies. These observations suggest two distinct mechanisms to regulate the cell size of diatoms, reduction and homeostasis. The interplay between these mechanisms can explain the behavior of different diatoms species in various environments.