I very much enjoyed staying at Juliette's studio. It was big enough for me, and I didn't mind getting some exercise climbing the stairs. The space is clean and artful, and the location is great. I got in too late to meet Juliette in person, but check-in was easy and all communications were cordial and efficient.
The shower (in the studio) and the shared toilet (in the hall) are tiny. At 6 ft / 185 lbs (1.83 m / 84 kg), I was ok, but if you're much bigger, you might not fit. Otherwise, you'll be fine with the space. All in all, a nice way to experience central Paris without paying a lot!