If you leave the location field empty, you will get a map showing the whole world. Otherwise, you should enter a natural number in the distance field to indicate the distance in kilometers that the map should cover. For example if you enter 8000, the server will generate a map showing the area 8000 kilometers around the center point. 1 mile is roughly 1.61 kilometers.

The maximum distance is 20015 which represents half the circumference of the earth in kilometers. For VHF operators, you probably want maps with a distance less than a few thousand kilometers.