Landscape size for a mobile game


Having some experience on vehicles, I’m planning to start a new mobile Project. I have some real nice height maps from real world and I use World Machine to modify and texturize them. The question is what is the optimum size for a landscape for a mobile platform. If I add a lanscape of 1009x1009 the tri count defaults to 2 million. The target landscape size is 8x8km2.