A friend and I stayed at Miguel and Sergio's airbnb for our first visit to Madrid. We we and to fit comfortably in the space. Air conditioning gave good relief from done of the warmth of Madrid in late September.
Not only is this place a great deal in terms of price, the convenience of its location pays off immensely. During our stay we rarely used the metro - any time we looked up a restaurant or activity we wanted to go to/do, it was a 10-15 minutes walk away. Made for a great home base and allowed us to take breaks without losing time.
Some nights, the neighbors play loud music or movies, more of a concern if there will be a light sleeper in the living room.
All in all, would highly recommend!