Explanation:

Platelets are the smallest of the three major types of blood cells.

Platelets are only about 20% of the diameter of red blood cells.  The normal platelet count is 150,000-350,000 per microliter of blood, but since platelets are so small, they make up just a tiny fraction of the blood volume.  The principal function of platelets is to prevent bleeding. 
Red blood cells are the most numerous blood cell, about 5,000,000 per microliter.  Red blood cells make up about 40% of our total blood volume, a measure called the hematocrit.  Their color is caused by hemoglobin, which accounts for nearly all of the red cell volume.  Hemoglobin is the critical protein that transports oxygen from our lungs to the tissues.  Red cells are normally shaped as round, biconcave discs.  With microscopic examination, they look like a red or orange tire with a thin, almost transparent center.
White blood cells are the largest of the blood cells but also the fewest.  There are only 5,000 to 10,000 white blood cells per microliter.  There are several different types of white cells but all are related to immunity and fighting infection.