The size of a cell is limited by its surface area to volume ratio. As cells grow larger in volume, their surface area decreases and their ability to diffuse compounds in and out of the cell decreases as well.
Cells diffuse nutrients and oxygen across their cell membrane to function properly and obtain proper nutrition. If a cell were to grow too large, then the surface area of the cell would not be large enough to transport adequate nutrition into the inside of the cell. In turn, the organelles within the cell would not get proper nutrition and would not be able to function properly. The cell's organelles would not be able to transport materials out of the cell properly.