First; You will not find a better or more dedicated host than Yannis. That's just a fact. He gave us so many suggestions and rekommandations for things to see/do/eat. Both from lists of general ideas/places, but also from current events (music, open air theater in historical surroundings...). We are so grateful.
The apartment is in a quiet street (or so it should be for many years - they just finished renovating the sidewalk the first days we were there), just under the old city, which is still in walking distance from the center.
It has two bed rooms with good space, a nice living room with access to a balcony, a functional kitchen, an entrance with good closet space. The bath room ceiling is a little low (I'm 1,80 and found a little bending needed, when standing up showering in the bath tub).
Yannis has given the apartment a very personal touch, with many decorative items with a history (that's at least my interpretation - meanin it doesn't look like an Ikea catalougue).
I highly recommend it. Thessaloniki is a charming city, the apartment is perfectly situated, and Yannis is a great host.