I just spent 5 days in Osaka at the Hostel Mitsuwaya as a solo traveler. The place itself is relatively small ( 2 floors + roof top ), but it just makes it easier to meet new people. The hostel is very clean and the staff is so friendly ! They gladly answered many of my questions (ex. restaurant recommendation, places to go out, directions, etc.)
I thought the beds offered enough room and privacy (keep in mind there are Japanese style beds that means you'll sleep on a futon instead of a mattress). The rooms are on the second floor while the showers are on the first floor, but in my opinion it wasn't a problem.
The location is very good : the neighborhood is quiet, however you can go by walk to Dotombori where all the night life takes place.
The only detail was that the breakfast was very basic and always the same, but it was free which makes it hard to complaint.
Overall, I am totally satisfied by my experience there, I would definitely go back or recommend this hostel.