there is no custom range for world map items. the only customization available for them in vanilla smbx2 is a feature where existing tiles will automatically be resized to fit the dimensions of their custom graphic.
there's also the script xmap
which will let you create txt files for overworld elements (like for npcs, blocks and backgrounds) allowing you to modify existing ones more extensively which can be used to compensate for the lack of completely custom options (however the aforementioned automatic resize doesn't work if you're using xmap, every parameter must be specified explicitly).
alternatively, you can forego use of the vanilla map altogether and go for smwmap
which allows for entirely new elements to be added (because it uses level elements which do have custom ranges) but has a bit of a learning curve compared to the vanilla overworld.