A key thing to remember is that in biology, structure determines function. In addition, we must also account for the fact that cells must constantly interact with their surrounding environment. That being said, cell size is limited by a cell's surface area-to-volume ratio. What this means is the amount of external material and organic matter a cell can acquire from its environment through the process of endocytosis. Thus, if the cell grows beyond a specific limit, not enough material will be able to cross the membrane in order to accommodate for the increased cellular volume. If this is the case, cells either have to divide into smaller units or face apoptosis. Therefore, it's necessary that the cells have a higher surface area-to-volume ratio in order to allow for more material to enter for the cell's metabolic process. After all, a small cell is more effective at transporting and metabolizing materials, as well as ridding itself of waste products, than a larger cell.
Although that's the main reason for limiting cell size, Nucleo-Cytoplasmic ratio, and Fragility and Durability of the Plasma Membrane are additional factors that limit the size. For Nucleo-Cytoplasmic ratio, one nucleus can only control so much of a region specific to cytoplasmic volume. Therefore, you may notice some cells vary in structure and may even be multi-nucleated because of this particular reason. Skeletal muscle cells and hyphae in fungi are excellent examples!
