What map has the biggest fields? Bonus points if the fields are flat.
Back Roads County 4x gives you 2 100+ acre fields if you start in new farmer mode. And there are numerous in the 80 to 100 acre range.
Be careful; one save I quickly abandoned tried to give me 117 acres of sugarbeats. The one I'm currently playing the 117 acre field is grass, which creates SO FREAKING MANY bales that my computer starts to lag when I'm near the stack, but it's better than root crops.
Another one I've played about 5 hours on is Spring Creek, ND. It's a 16x map, and one field I can remember is something like 217 acres. I abandoned that because it has too many contours on the edge of the field that Course Play just about bricked my computer before deciding it couldn't generate a course. I decided I didn't want to spend a week IRL just plowing a single field.