This question has been asked thousands of times, but generally the answer is that size is limited due to the need to have sufficient surface area for its volume.
Understand that as a sphere increases in volume, its surface area also increases. This is pretty logical, right? However, the volume increases at a faster rate than the surface area increases. This is because the surface area = 4pi(r^2), whereas the volume = 4/3 r^3. Volume increases as the cube of the radius whereas surface area increases as the square of the radius. So, you can see how the ...
Balances in volume and growth are discussed.