Our family of four, including two kids (9 and 12) fitted into the apartment just fine using the sofa-bed. The kids even commented on how comfy it was. Having two bathrooms made things really easy. Wifi was excellent and stood up to the YouTube demands of the kids after a busy day sightseeing. The apartment is up about 3 flights of stairs so it's good exercise! You are hidden away from all the noise of Rome and we found it very quiet.
Our host Stefano couldn't have done more to help us, explaining everything about the apartment, providing a map of his own recommendations for restaurants (we ate at two and enjoyed both), booking collections from the airport and back again, he even carried the biggest of our suitcases up the stairs.
As other reviewers have said this apartment is perfectly located for all the main sites. It's possibly to walk everywhere but easier in the July heat to grab a taxi from the Pantheon rank just a couple of minutes walk away. Getting back to the apartment we always found it easy to direct taxis to the Pantheon and then walk the last hundred yards or so, normally grabbing a few things from the mini-supermarket (excellent store!) on the way before relaxing in the apartment with the excellent air-con to cool us down.
We can't recommend the apartment enough. So much better than staying at a hotel in our opinion! Normally you walk past all the doors to city buildings but with Stefano's apartment you actually get to see and live inside! We loved it.