OME can make large maps, but it doesn't have the terrain generation that the server itself has. So there are a couple ways to go about doing it.
Use a height map to import. Just be careful with this one, because sharp gradients cause high cliffs that are rather annoying for building.
Draw your own land using the tools. Obviously, this would be really, really tedious, but it would work exactly as you'd want.
Export the layers from a normal generated map, then double their size and add them back into a 512x512 (which would be double normal) size map. Also tedious, but it would look mostly like a normal map.
I'm just personally hoping for some map generation options in the server software soon. Y'know, map size, terrain options, good stuff like that.
That wouldn't be too hard - for making a 256x256x256 map, you can just stretch a regular map using Skrylar's map filtering tools: a regular map is 256x256x64, and Skrylar's tool can double the height (twice). The only problem is that there won't be any detail below the original bottom of the map.
Increasing the height of a regular map  to 256 would leave you with 4 times the regular depth of earth and stone ... 3/4 of which would be solid so, if you enjoy the caves, that's a lot of lost exploration.