We had a wonderful stay at Masa's and Borja's! We were 4 adults, 3 kids and an infant and the place did not feel crowded at all. The apartment was very clean with everything we needed. The flat is very well located and within walking distance of all major spots. Access to the balcony and a parking spot was a definite plus. We had an issue with the keys which was resolved very quickly. Watch the sidewalk grid by the front door of the building and be careful not to drop your keys in there... this happened to us at night and we had someone there to let us back in super fast, within 30 minutes, after calling the contact number (I'm so sorry I woke up Jose).
I highly recommend this place in San Sebastián!