The cell's size totally depends on its need to pass the nutrients and gases into and out of them. Cells need fast and easy food. If the cell becomes too large, it is more difficult for the nutrients and gases to move inside and outside from them. That is why the cells are small and no organism have a huge cell.
As the cell grows, the volume area increases more rapidly compared to its surface area. When this happens, the small surface area would not permit enough nutrients and gases to enter the cell to sustain the cell's needs.