Brooklyn College explains that cells are small because they must have a large surface area relative to the amount of volume they contain to function properly. As a sphere grows larger, its volume increases much more rapidly than its surface area does. This presents logistical problems for the cell, as it tries to transport resources and products through a large volume without the resources available via a large surface.
As an example, a typical animal cell requires oxygen to survive. The size of the cell partially dictates the amount of oxygen it needs, while the surface area of the cell limits the amount of oxygen that can be absorbed at a time. Accordingly, when the size of a cell grows, its demand for oxygen and other resources rises at a rapid rate, while its capacity for absorbing oxygen increases more slowly. At some point, the size of the cell will cause the cell to divide or die, according to Brooklyn College.
Despite the limitations on cell size that are imposed by the surface-to-volume ratio, a 2013 study published in the journal Nature Cell Biology demonstrates that gravity also limits cell size. While gravity is a negligible force at very small scales, cells that attain about 1 millimeter in diameter must include structural elements to keep some organelles stable under the influence of gravity. Without such elements, cell components can lose their structural integrity.