The place is really good. We were a group of 7 and it was able to fit us comfortably. The location is great, pretty close to Nishishinjuku Gochome station, and close to Shinjuku main station.
If you carry a lot of bagagge our heavy stuff you may know that the stairs and the access can be a bit hard for some people, but nothing that matters too much. The only thing I can say to improve the place is to put some indications in english in order to use some of the devices in the house (for those how don't speak japanese).
Overall, Shinju's place is very good and we felt really well!