But generally, cells tend to stay in the 10 m range. Cells carry out metabolic processes through their cell walls. Metabolic process are all the chemical processes that occur at the cellular level that keep a cell, and therefore the whole organism alive. Protein synthesis, respiration, digestion and waste removal are examples of metabolic processes. Although a cell is small, it has a complex molecular composition.
Molecules can range in size from a few nanometers to a few micrometers.
Molecules inside and outside the cell, such as carbohydrates, enzymes, hormones, nucleic acids must interact and react together. If a cell a were not limited to a size of only microns in diameter, then it would be more difficult for the various and sundry organelles, molecules and even individual atoms to come in contact and do their stuff. The amount of metabolism a cell does is directly related to the volume of the cell, however, the ability to do these things is limited by the surface area of the cell. Since the surface area per volume decrease as objects get bigger, there comes a time when a cell is unable to get any larger. As things get bigger their volumes increase rapidly, but their surface areas don't increase as quickly.