No, I've never seen a 100 acre field of watermelon. It'd be quite a sight.
I'm not sure I have either, but the fields in Southeastern Missouri when I was a boy were immense, and there were lots of them. The land there was perfectly flat.
I lived across the river in Kentucky, but one of my grandfathers lived in a small town not a terribly long way south of Cape Girardeau, home of the inimitable Rush Limbaugh.