The size of a cell is limited by cell’s surface area-to-volume ratio.
A cell differs not only in its shape but also in its size. While a few cells are large enough to be seen by naked eyes, most cells are microscopic in size.
When a cell grows, its volume increases faster than its surface. Nutrients and oxygen needed by a cell enter a cell and wastes such as carbon dioxide exits a cell at its surface. Large cells require more nutrients and produce more wastes than small cells. Thus it is obvious that the greater the amount of surface area, the greater the capability to get material in and out of the cell. If it is a very large cell, then the volume has increased much more than the surface area.
However, when cells become larger in volume, their proportionate amount of surface area essentially decreases. Consequently large cells are not capable of exchanging nutrients and wastes perfectly due to the decreased surface area-to-volume ratio.