Had a lovely stay at Marta's apartment in June. The flat is big, comfortable and very well equipped and I would say is at least as big as the pictures make it look.
It's on the top floor of a nice and quiet street, which means a lot of stairs but as long as you know that in advance you'll be okay but if you have any mobility problems might be a struggle. By metro it's 5 minutes to Ramblas with 2 metro lines in a few minutes walk of the apartment, and only 20 minutes to walk to playa catalunya. The very beautiful park ciutadella is only a few minutes walk away. There's a (large) supermarket within 50 metres and bars and tapas all over around the area, as it is all over Barcelona.
We were greeted by the very pleasant Marta on arrival who showed us around, and her friend was on hand to help us check out at our preferred time. The place itself has excellent wifi, air conditioning and a small balcony just about large enough to put a single chair out onto. There's an electric hob with 2 rings and a large fridge freeze, as well as a washing machine. It's big enough for 2 people to stay in, sharing the double bed comfortably but I don't think any more.
We found Marta very accommodating and professional and had a lovely time here. I would definitely recommend this apartment for a single traveller or couple :)