Individual cells are usually very small

Because the people that mae them wanted them to be very small

They are usually small because they need to transport nutrients across the membrane. Only particular amount can cross per/area. As the volume of cell increases, it increases in geomatrically faster than the area of the membrane.It becomes difficult to a certain point to supply the given volume with enough membrane to allow the gases/other molecules to transport across the membrane

:)

The answer is 0.01 meter