This is something no one else except you can answer, at least until you have players testing your experience. A reasonable size for a map depends wholly on the experience.
Is the game adventure based? Will new players expect a large area to explore?
Does the size of the map fit with the gameplay? Are players roaming around forever just to get where they need to be, or are they always able to be occupied with something.
Is your map and props optimized? Can everyone regardless of device play your game. Maybe only certain devices would be capable? Or perhaps it’s completely unplayable and too laggy when you add more than a few players.
These are questions best asked when designing an experience before you start building a map. Having the foundation and core loop of your experience is really important before moving onto creating a map or modeling assets. I would suggest you really delve into what your experience is about, expand upon that, and then evaluate your map based on what you learned/came up with.