Skimmer compartment size in sump

Hi Everyone. I'm in the process of designing a new sump for my 400 litre marine system. I'm after advice on the size of the compartment to house my sump. I'm using the TMC Reef Skim 500, Dimensions - 420mm (H) x 190mm (L) x 160mm (W). I've read that skimmers work best in certain size compartments etc. Any help would be much appreciated. Phil