I'm not sure how big a sump should be for a fresh water 1000 liters of tanks. The main issue I could see is it just needs to be big enough to hold the pump/s heaters, Bio media/sponge filter and can handle the amount of water flow thru the sump. It also needs to be big enough to have enough room for all of the water to flow back to the sump from all of the tanks when the water pump/s are turned off. For reference Saltwater aquariums usually have the sumps about 1/3 the size of the main tank.
Drew..