Looking for some general feedback and ideas before undertaking creating a new world. What I am trying to do is create a very realistic world map for a fantasy world. The world will be approximately the same dimensions as earth. The diameter will be 16,200 leagues. I am contemplating creating a map 16,200 pixels by about 6300 pixels which would cover the entire globe east to west and to about the 70th parallel north to south. I realize that the map will be stretched at the north and south but I'm not concerned with that. I do plan on having at least 16 climates using rainfall and wind patterns to make it very realistic. That part is no problem either. There will be at least 11 major races and many minor ones with a total population of about 400 million. I could make a good looking map with a lot less pixels but I want to place a lot of population centers and want them to be relatively realistic, There will be lots of borders and rivers etc. Since a league is about 1.5 miles even at the resolution mentioned every border I draw is a mile and a half thick. I plan on using GIMP and am getting warnings about file size when I create the new file.

Please share your thoughts maybe there is a much easier way to do this.