For a perfect sphere it is quite easy to determine the particle size: You only need the diameter. If all particles were spherical it would be quite easy to determine the particle size using the available methods. Unfortunately, molecular systems very seldomly form nice spherical particles. Instead, they form various other shapes – and all particles in a mixture have different particle size.
Before you perform a particle size analysis you should be very careful about what method you use – and be aware of the strengths and weaknesses of the various methods:
There is not such a thing as a perfect description of particle sizes, but a combination of methods can give you a reliable description in order to control your product. Go to particle size determination to get a summary – including strengths and weaknesses.