I don't know from an addon, but i know that WEU or NewGen can make bigger maps like the size you wrote.. so download these World Editor Tools from the Tools section and create a map with it.. it's legal to use ^^ but there could be some errors or some strange bugs due to custom tools.. so make sure to save often..
Another way would be reducing the size of your models/doodads/etc on the map.. make it tiny, zoom in and lock the camera so the normal player will not notice any changes.. and you can easily use a normal sized map.. just take smaller doodads for it (i would suggest this)