My family of 4 rented an apartment just off the Puerta del Sol over the Christmas holidays. You could easily accomodate 6 if the living room sofabed were used. There are two bedrooms with double beds and two full bathrooms. The living room and dining room areas are much larger than that of the apartment you have posted about. We rented through
VRBO . I see that it is the same owners for my apartment and the one you are looking at. Fro 50 Euro more, I think you will get a lot more space. The apartment is the VRBO #66092 called From Madrid to the Heaven.
Just another option...
Eve