remember, the 32 by 32 heightmap that starts in an area is just an arbitrary landing pad. You could duplicate it and stitch, use the grid panel to make a mile by mile heightmap, import a heightmap, whatever. A ten mile by ten mile zone is most likely a terrible idea to start with, unless you have seriously experienced world builders that have decided on that space for specific reasons. Most inexperienced teams start with world building, since that's the easiest thing to do, and doesn't require technical resources, but truthfully, world building comes in very late in the development process compared to art and engineering in most successful projects. Now, what's great about our engine is that on day one you can start building worlds, but I would focus solely on prototyping areas and zones while you get your systems and art hammered out. Art and engineering and system design should be driving world building and content creation, not the otherway around, of course some will cry havoc here, but I'll stand by that statement. I've shipped too much product and watched too many projects fail to think otherwise.
I say think small and fun and tightly polished first, then expand when it makes sense. No one has ever bought a game, or invested in a game because it was huge. World of Warcraft has effectively a 6 man dedicated team for each zone creation process, and even then, some clearly succeeded in look and feel and gameplay, and some still failed miserably.