How are you guys going to work with the overworld areas?
The format is already chunked so right off the bat, the engine won't be loading the entire map, but only a small region. Secondly, we're going to skip drawing anything that's beyond a "render-safe" zone, so in other words we're doing renderculling. These are pretty standard tricks. The size of the maps should be of zero concern for the designers because the engine will be able to compensate for the size.
That being said, I want to acknowledge that I've rigged the map format to only be able to do one tileset per map. Each chunk can have different tilesets, but this is only for exceptional cases such as the Overworld.
Should a map be submitted and it rely on multiple tilesets, there's a chance it will be shot back at you for to revise. The reason why I say this is because, it's not optimal to simply do that. Will the overworld require multiple tilesets? Yes. Is that fine? Yes. What about a dungeon? Absolutely not. These are considerations that have to be logically factored in.
At this current time, nobody has to really worry about it. This only applies to when we release the editor, which should be coming up soon.