Small raindrops are more spherical than larger ones. The frictional drag from air causes large drops to have a hamburger bun shape. The volume of a sphere is 4/3*Pi*R^3. The volume of a small raindrop can be approximated by using the sphere formula.
Example: What is the volume of a 0.2 cm radius rain drop?
4/3*Pi*(0.2)^3 = 4/3*Pi*0.008 = 0.034 cm^3
Example: How many 0.04 inch radius raindrops can fit is a square container that is 1 foot on its side and have the entire volume of the container occupied?
0.04 inches = 0.003333 feet
4/3*Pi*(0.003333)^3 = 0.000000155 feet^3
1 foot^3 / 0.000000155 feet^3 = 6,445,775 raindrops