What are the different airsoft bb sizes?

It depends on the size of the BB. A standard BB gun shoots a .177 caliber ball. The diameter is 0.177 inches. The formula for volume of a sphere is 4/3 * pi * radius^3 .177 is the diameter, so halve it it to get the radius. 4/3 * 3.14 * (0.0885 x 0.0885 x 0.0885) = 0.00290201 cubic inches.