The apartment is actually quite big. It's well equipped, the beds are very comfortable/big, and the place has a modern look. It's 10 min away from the closest metro station (L4, Poblenou station) and almost right next to the beach, so even though it isn't located in the center of the city, it isn't that far away either.
My only recommendation for traveling to/from the airport (El Prat) is to take an Uber from Plaza Espanya (or the airport itself). Even though Barcelona has great public transportation, you have to take 1 bus, then two metro lines (you have to switch between then, just follow maps), and the walk 10 min from Poblenou's station to the apartment, which takes around 1 hour. It doesn't sound like a lot, but the metro has lot of stairs, and the walking with luggage for 10 min can be quite challenging.