Really cute apartment, designed well for the small space. Really great location about a 10 minute walk from Atocha train station and Reina Sofia which positions you just a little south and east of all the major tourist spots. It may be small but what you need is there - fridge, kitchen utensils, hair dryer, end tables and even a computer bar with stools. The shower/tub is tiny and you need to step up into it so make sure that isn't a problem for you.
It has AC but I wouldn't stay here during a Madrid summer as, if you're American, it's not the AC you're used to. I think that may just be European AC as I stayed in 2 hotels in Madrid and Barcelona (one was a four star) and it was the same. When we stayed we used the AC and the open balcony windows which let in the beautiful early May night air to cool the apartment down at night (we weren't there during the day) then closed the windows once it was cool. Note: I probably wouldn't go to Madrid later than early-mid May until it cools down in the fall anyway.