How to Size a Cyclone or Hydrocyclone has a well establish step-by-step process for which Richard A. Arterburn was a pioneer. Since this paper and its hydrocyclone sizing method is becoming hard to find, I uploaded it on here.
Use the 911Metallurgist Online Cyclone Design Calculator.
https://www.911metallurgist.com/hydrocyclone-design-equations-and-sizing-parameters-calculations
I also include an Excel file prepared by Alex Doll from SAGMilling.com to help you properly calculate the size of a hydrocyclone.
Further credit FLSmidth
Discussion classifications by screens and cyclones.