As the cell size increases, cell volume (a cubic function) increases more rapidly than cell surface area (a squared function).    Check out the cubes in your study guide (objective 4) to verify this.   This is important because of a rather simple minded fact - the materials needed by the cell contents must enter the cell across its surface and waste products must leave the cell at the surface.  Thus if the cell becomes too large there is not enough surface to support the needs of the cell contents.  In short, cell size is limited because the relationship between surface area and volume becomes less and less favorable as the cell size increases.