Location, location, location!! This apartment was an absolutely perfect location for our Paris stay! As described by the owner, it is small, and it is up 5 flights of stairs (which is a perfect rationale for having croissants every morning for breakfast). My husband and I vacationed with another couple, and found the apartment ideal for what we wanted - a central, comfortable, clean place. One of our favorite meals we had in Paris was cheese, bread, and wine on the balcony. Very close to the Louvre (2 blocks), close to the subway/train station (about 3-4 blocks), lots of cafes and bakeries nearby. Definitely take the short walk to the park and shopping/eating district (the Les Halles district, the park is about 2-3 blocks, the shopping/eating area is just beyond).
Regarding the size: the upper bedroom has slanted ceilings, so it feels smaller than it really is, but it has it's own full bathroom. The main floor bedroom is probably smaller floor space, but feels bigger. The main level bathroom is a little unusual - the toilet (which has a tiny sink at the tank!) is in a small closet size room just at the base of the stairs to the upper level, and the shower and a sink are just off the kitchen (separated from the kitchen by a curtain).
The beds were comfortable, the apartment was clean and charming.
The owner, Quentin, was incredibly responsive to any questions I had (usually a response within minutes!).
Don't let the small size and the stairs keep you from staying here, the benefits of the location are worth it.