Answer : Cells cannot be as large as a cube because of the surface area to the volume ratio gets smaller as the cell size gets larger. So, if the cell grows beyond a certain limit, the cell contents will not be able to cross the cellular membrane that fast, to accommodate the increased cellular volume.
Also, cells are so small in size, so that they can maximize their ratio of surface area to volume. In that case, smaller cells will have a higher ratio which will allow more molecules and ions move across the cellular membrane per unit of cytoplasmic volume.
Therefore, cells are so small in size as they need to be quick to get the nutrients in and throw the waste out.
