Well you can get into the 1k meter range per area. Just need to figure out some options to break the gridding up. It's always a challenge with a flat regional area. It doesn't always have to be a mountain, it can be as simple as a wall that separates an area's areas off, or even a large building that blocks out the corners, etc. Goal is to minimize 4 corner dancing and proxy loads.
It's always a challenge to make things happen, and a judgement. If you want to build super computers and pay for high bandwidth costs, and trim large chunks out of your profits anything is possible. Just be aware there are some caps to how things work in regards to server processes. Also depends on how combat is going to happen, how much server data has to be maintained / processed etc. Also could look to limit number of people in an area. I know seems horrible but players don't mind it much. Cause having to split up a bit into instances, versus having unplayable game is a pay off most gamers will balance. As long as you keep things even and fair.
So it's always a juggling act, but just be mindful that fighting puts a bunch of load on an area, and adding data from 4 other areas tends to create a bunch more processing. As each area has to keep tabs on all the data from the 4 other areas.
Also just get creative, a fort can sit on just an area big enough to contain it, and it only has to connect to 1 area, aka the entrance, everything inside it can be cut off from the rest of the world. Requires a bit of double area work, but something that can make large areas, without actually having "large" areas.