Sonya was absolutely lovely--a born host for travelers. (We never met Artur.) We gave her five stars, and I checked all the elements Airbnb lists in their 'what was special' about her arrangements. I wouldn't normally do that in a review, but in Sonya's case, it was certainly all true.
While the studio apartment was fairly compact, it was very well laid out, and completely immaculate, with modern appointments in kitchen area and bathroom as elegant as I've seen, and very comprehensive in the kitchen. (And I've used airbnb a fair bit over a period of years.) The only drawback, I suppose, is that there's really no good space in which to install an easy chair of some kind. But it was absolutely fine for our purposes, with a very comfortable mattress, and a pleasing view of the street below us and the houses across that street, from multiple windows, which you can shutter closed on the inside if you wish, to cut down on street noise or morning light. (It's a little side street, so not too noisy even on a week-end night.) We would certainly stay with Sonya again, were to make our way back to Lisbon sometime (which we both loved).
A couple of additional considerations: Studios of the Panteão are quite literally a 5 minute walk (downhill) from the Estação Santa Apolónia (major long-distance train station near the city center), which also houses a metro terminus on one of Lisbon's metro lines. You're in the Alfama, so walking around there and Graça is your neighborhood. Several other center city neighborhoods are also within easy walking distance, if you're moderately energetic. Second, Sonya's and Artur's rental rates are quite competitive, at least to judge by my own little window of research about a month before arrival--a great place to stay.