Large open world landscape

Thanks for getting back. I am looking to create a vast map with a desert wasteland and a city that is way past the desert limits, think Fallout and other RPGs, I would like to know the steps to create mpas along those lines. With target platform, I would like it to run on modern consoles.

Roughly what size is a typical open world game? Say on the lines of Fallout or others in that region.