The draw_minimap function scales the drawn map based on how large of a region you read data from, you can draw a larger map by just reading more data:
var plist = worldmap_get_visible_room_data(0,0,30,20); draw_worldmap_segment(plist,0,0,VIEW_W*0.5,VIEW_H*0.5); ds_list_destroy(plist);
(Basically, coordinates will be scaled to a 0-1 range in the read functions and then the map-drawing functions will stretch that range over the region you draw over)
(The reason it's split in a read function that generates intermediate data, and a draw function that uses it, is because we need to check all rooms for data and it's a slow process, so we only do it a couple times per second (instead of every frame) using an alarm)