if you're using the vanilla world map, you can sort of have more tiles by familiarizing yourself with lua scripting and either downloading
xmap which lets you manually draw arbitrary graphics onto the world map, or using the sprite override fields in the graphics class to change tile images on the fly
if you don't want to script there is also the option of pre-rendering your overworld by putting the tiles together in an image editor or other external software then exporting it as a set of large images (smbx2 should automatically resize tiles to match their graphics), but this could end up being cumbersome