The functions of the cell are generally thought to determine the upper and lower limits of cell size. The determination of cell size has been a matter of inquiry by scientists for a century or more.
The cells must be large enough to include all of the necessary structures and materials, including DNA, ribosomes, enzymes and other cellular structures, determining the lower end of the scale. On the upper end of the scale, it must not be so large that it can no longer control the enzymatic and cell division processes that are essential to its functioning.
An individual cell must also be the right size to fit into whatever larger structure or substance it may be a part of as well as the environment in which it exists. Some experimental evidence suggests that the cell itself is aware of its size and can regulate it to time cell division.