Must be a rectangle sorry i didn't clarify thatIt's impossible to get that if you don't include a jungle, snow, desert, or swamp biome. Even if you include the ocean, people have built random little houses all over the ocean.
You're not going to get a region of 855x640 blocks without hitting one of those biomes or someone's house or projects, you can't be too picky otherwise you're building in the ocean.
That, or you're not going to get a rectangle one, I found a decent area right here:
http://smp.opticraft.net/map/#/-5931/64/4175/-4/0/0(click to show/hide)