My husband and I stayed in Diana's studio for three days during our honeymoon, and we had a great time! It is "intimately sized," to be sure, but we spent most of our time outside (also because, being late summer, it is quite hot inside the apartment) and the neighborhood can't be beat. It was easy to walk all over the city, including to museums, the train station, bars and restaurants galore.
Diana was there to show us the apartment, explain the keys, etc, and gave us some great tips. Added bonus: washing machine!! It was a great place to stay and explore Madrid.