a sump should be at least 25% of the mains total gal
to be effective.
i.e. a 75 gal
tank should have a 18.75+ gal
sump so a 75 should be able to accommodate a tank up to around 220 gal
Of course the bigger the sump the better but having at least 25% of the mains total typically will give you enough room for equipment and baffles and is able to handle the overflows max volume of water.
You will probably only get about 50-55 gals of water usage out of the 75 using it as a sump with it 2/3rds full during operation to allow for drainage back from the main.