I used a 10A 20mm fuse. There is a fuse drawer included with the module where the power cord plugs in.
The relays' max current spec is 10A so that's the main reason I chose this value.
Also, I don't expect the current draw for the controller to be more than 10A even with heater, filter, lights, skimmer, etc all connected and on at the same time. If for some reason my controller is drawing 10A at 120V, then there's a huge problem happening somewhere lol.