Cells take in nourishment through their walls. As they grow in mass, they require more nourishment, but the surface area increases at a slower rate than the mass does. Therefore, if it gets too big, it will have trouble taking in enough nutrients and will starve. At that point, the large cell divides into 2 smaller cells. At the cellular level, whales are similar to mice, and the upper limit in size will be similar.
Work Step by Step
This is discussed on page 239.