There are two main methods to select an active harmonic filter.
1) Know the plant operating details: Could be from continuous monitoring (already have appropriate metering installed); could be from performing a harmonic study; or could be from performing a simulation. All provide details about V (phases and neutrals, if appropriate), I, KW, KVAR, KVA, PF, THDi, and THDv. From this 9information some good sizing can be attained.
2) If planning/designing a facility, simple calculators provided by the active harmonic filter (AHF) manufacturers allow entry of load sizes and types and utility/generator source information. This is more conservative than point 1 above. But it permits one to know what will meet the harmonic standards and protect the eventual users.
You need to define the starting point and decide what the standards of compliance to be applied.