1. Diffusion limits cell size. Diffusion becomes slow and ineffecient as the distances become larger.
2.DNA limits cell size. There is a limit to how quickly the blueprints for proteins can be copied in the nucleus and made into proteins in the cytoplasm. Cells cannot survive unless there is enough DNA to support the protein needs of the cell.
3. Surface area-to-volume ratio limits cell size. As a cell's size increases, its volume increases much faster than its surface area, resulting in the plamsa membrane not having enough space thought which oxugen, waste, and nutrients can diffuse. This would result in the cell either dying of starvation from lack of nutrients, or poisoning from excess waste.