Bigger Tileset
 

Does anyone know how to make the tileset bigger, if you can? And how to make the tiles like solid tiles or water tiles, if you can?

I dont think it can be done. Because when you use addtiledef stuff if you place the image over a non-walkable tile it's non-walkable same for walkable, so apparenty Stefan has it coded that only certain tiles you can walk on and a new extended tileset would mean a new version of the graal.exe if I am correct in saying so.