1. How Many Cells Are in the Human Body???
About 100 Trillion

2. Surface Area to Volume Ratio
The surface must allow sufficient exchange to support the contents of the cell.
As these cubes illustrate the surface area to volume ratio of a small object is larger than that of a large object of similar shape. This ratio limits how large cells can be.

3. Compare the surface to volume ratio (surface:volume) of a cube that is 1 cm X 1 cm X 1 cm with that of a cube that is 10 cm X 10 cm X 10 cm.

4. The surface area of one side = 1 cm X 1 cm = 1 square cm (or 1 cm2).
Smaller cube (1 cm X 1 cm X 1 cm)
The surface area of one side = 1 cm X 1 cm = 1 square cm (or 1 cm2).
There are 6 sides, so the total surface area = 6 X  cm2 = 6 cm2.
Volume = 1 cm X 1 cm X 1 cm = 1 cubic cm (or 1 cm3)
Surface:Volume = 6 cm2/1 cm3 = 6 cm2/cm3 (or 6 square cm of surface area for each cubic cm of volume)

5. Larger cube (10 cm X 10 cm X 10 cm)
The surface area of one side = 10 cm X 10 cm = 100 square cm (or 100 cm2).
There are 6 sides, so the total surface area = 600 X  cm2 = 600 cm2.
Volume = 10 cm X 10 cm X 10 cm = 1000 cubic cm (or 1000 cm3)
Surface:Volume = 600 cm2/1000 cm3 = 0.6 cm2/cm3 (or 0.6 square cm of surface area for each cubic cm of volume).

6. Notice that the larger cube has more surface area and more volume but less surface area for each cubic centimeter of volume.  
For any given geometric object (cubes, spheres, etc.), smaller objects have a greater surface to volume ratio (surface:volume) than larger objects of the same shape.

7. How Surface Area to Volume Ratio Limits Cell Size
A cell is a metabolic compartment where a multitude of chemical reactions occur.
The number of reactions increase as the volume of metabolic volume within a cell increases. (The larger the volume the larger the number of reactions)
All raw materials necessary for metabolism can enter the cell only through its cell membrane.
The greater the surface area the larger the amount of raw materials that can enter at only one time.
Each unit of volume requires a specific amount of surface area to supply its metabolism with raw materials. The amount of surface area available to each unit of volume varies with the size of a cell.
As a cell grows its SA/V decreases.
At some point in its growth its SA/V becomes so small that its surface area is too small to supply its raw materials to its volume. At this point the cell cannot get larger.