For a 42k gallon pool, NSF recommendations are for a 60 sq-ft filter. However, a 1.5 HP Superpump should not deliver more than 70 GPM on 2" plumbing, which means that you could probably get away with a 36 sq-ft filter but to make sure that you are below the 2 GPM/sqft limit, I would leave at least 25% margin and go with a 45 sq-ft filter.


But just to make sure, can you describe the following:

Height of pump relative to the water level
Suction pipe diameter (e.g. 1.5" 2", 2.5")
Number of individual suction pipes from the water to the equipment
Average length of suction pipes
Number and type of suction side valves
Filter PSI reading at the desired valve settings and pump's highest speed setting
Heater - yes or no