We really enjoyed our stay in the apartment!
It is indeed very small but for a few days as a couple it was great. The bed is super comfy and the bathroom even has a small bathtub. Also the kitchen has everything we needed. A great advantage was also the washing machine that we could use for free. For us it was a bit weird that there are many signs in the flat stating rules and restrictions, but that seems to be normal for Japan :D the metro station was just a 3 mins walk away and we could reach every destination in Tokyo from there very well.