Sabrina's place is in a great central location, with a rare view! It was completely comfortable in terms of size and temperature (there is air conditioning which functions well if you need it, but in late September the windows open was enough for me). The shower is very large and there are plenty of spaces to relax or sit (bed, lounge chair, kitchen table stools). I ended up needing to do some work while I was there and the wifi worked well for a video call. Sabrina was easy to coordinate with for check in and offered a late checkout. She was super friendly and I felt very comfortable as a solo traveler.
Everyone seems to comment about the steps. For me, the steps were no problem and if you are sight-seeing around Florence I guess your typical day at a museum or garden would involve many more steps than Sabrina's ;) But if you have a big or heavy bag it might be unpleasant to carry it upstairs. Or if you have balance issues, the multi-level layout of Sabrina's flat could be difficult. One other thing to note is that in the evenings there is music playing in the square or restaurants below. I found this interesting to listen to but if you are sensitive to noise it could be irritating. It was over by the 11pm quiet time and I had absolutely no problem sleeping. Note that church bells ring in the morning at 730am. I don't think the music or church bells would be avoided anywhere in central Florence so these should not deter you from staying at Sabrina's - just something to be aware of.
Florence is beautiful - enjoy your stay at Sabrina's!!