It's 1.5 kg.. I had to break the original rocks with a hammer as they were too big for the tank. I found lots of bristle worms and crabs inside.
Anyway, I believe there were even smaller tanks on nano-reefs.com.. that's where I got the inspiration from. With something this size there's no room for skimmers, so I'll rely on water changes. I will not be using fans, as evaporation in this size will be drastic. I plan to minimize heating by raising the lamp high above the surface.
Polyps on the live rock have started to open already. I hope that's a good sign.