I recently stayed here with a group of 9 (7 adults and 2 infants). It is a really big space (for Japanese standards) and we didn't feel cramped at all. The kitchen has all the amenities you would need for a lengthy stay in Tokyo, we would regularly shop at the nearby 7-11 and store our goodies in the huge fridge :).
The location is fairly convenient. It's a good 6 - 7 minute walk from the train station but this is on residential streets which are never busy. There is also a bike path nearby which almost leads directly to the station. its close to 2 main trainlines; Nakanosakaue St - JR Marunouchi and Nishi-Shinjuku-Gochome - Toei Oedo line) meaning you can easily get to or from Shinjuku station or Tokyo station. Both trainlines have lift access to the tracks, which made getting around with prams super simple.
There is Airconditioning in all rooms, even on super hot summer days we were very comfortable in the house! There is also a washer and lots of space outside some rooms and on the roof to hang clothes.
Overall, I wouldn't hesitate to recommend Shinju's place for larger groups of travellers.