How big can the maps be?

Title says it all, just curious. Becasue every map in this game are like small islands.

 

there is no "hard" restriction on it.

The game uses heightmaps for the main terrain. These heightmaps are greyscale images which can be up to 4096*4096 pixels. The more white a pixel is, the higher the terrain in game at that point is, less white = lower terrain. You can set how far apart these points are though. Default I think is 1 pixel = 1 metre. So on a 4096*4096 pixel height map at 1pixel per metre, you will have a 4096m wide map and the smallest bump you could represent would be 1m across. In theory you could set it up so 1 pixel is 10 metres in which case you have a 40km wide map, but it wouldn't be smooth at all. At the same time, you could import so 1 pixel was 0.1 metres and have an incredible amount of detail to work with.

That is the real restriction. You can only go so large before suffering from resolution issues.

The torque3d game engine (which BeamNG uses) does support tiling multiple separate heightmaps edge to edge to bypass that limit, but BeamNG doesnt simulate collisions on the extra tiles yet.
Another workaround is to simply place a mesh object in game as part of the terrain. Can be more difficult and sometimes more processor intensive, but it can be used to have a fairly low resolution heightmap for a gameworld of the size you've chosen with special high detail sections where required, tends to work best for man made stuff like ramps etc (gridmap for instance, all the objects to crash into there are not in the heightmap and are separately placed objects.).