There were 13 of us (8 adults, 4 kids and 1 baby) and one of the prerequisite was to find a place with 2 bathrooms and 2 toilets. Masato's place was perfect. The house is located in between 2 stations; Izumi Tamagawa (5 mins walk - supermarket & restaurants) and Komae (15 mins walk - 7Eleven & restaurants). Eventhough on average it took us about half an hour to reach Shinjuku (on local train - we opted to use this as we did not want to change train with kids in tow), the journey was pleasant (we avoided rush hours).
The house was really clean and very spacious even for 13 of us. We rented 2 cars (minivan & sedan) for a few days during our stay and they fit nicely on the drive way.
Masato lives just next door; check-in & check-out was a breeze. Do pay attention when Masato explain how to use the appliances as they are in Japanese.
We had wonderful time staying at Masato's place and it is highly recommended for big group. We would definitely love to come back again.