In addition, the substances that cells work with are already really small, both ions and very small molecules. But you cant decrease the size of these things, the cell has to be able to support these ions and such. Scientists have the ability to detect something as small as an electron. Electrons are the smallest part of an atom. Since every molecule as at least one electron and it takes several hundred to even in the thousands to make cellular material the assumption is able to be made that there are not cells that are too small to detect.
Cells are not larger that 1000 um because of the structure of a cell it wouldnt be possible to make a large ball of thickly solute fluid out of a skin of fatty membrane. The cytoskeleton is built to just take on a certain amount of space size wise, it could fall apart. In addition. Genes and hormones tell how a cell is made, the factors serve to regulate the cell size and prevent its rapid and undifferentiated growth. For example, cancer is what happens when a cell keeps growing. Genes are so smart that they make the codes for cells and its job is to make the codes that keep the cell functioning. Genes evolved to find the best cell size that would give the best size, energy, and structural features for a particular organism.