Making a 16 giga-pixel map with 128 DM can take a week or more to generate (depending on the erosion level). If you use a higher detail DM of 32 and 64 it can take a month or more.
SO, Depending on what you’re trying to make, a map this size is for all intended purposes useless since the data size is 32 giga-bytes. In some engines like Esenthel and Bigworlds this can quadruple as the terrain texture alphas get plugged into the terrain data chunks. The only use of this map would be rendering high Q stuff with resolutions 1 meter and under. If you want a very large map and want it in a hurry then use a smaller generated map and export it resized. Of course you loose some detail this way but the process is a lot faster.
example: I want a 16 giga-pixel map (131,072) with a resolution of 1 meter per division.
I generate a 32K DM with a DM ratio of 32 and Horizontal resolution of 4 (takes two days on my box which is a quad core 3.0 GHZ)
export it as a 131072 HF which will equal the same details as a design map for 131072 at DM ratio of 128 and a horizontal resolution of 1. if you want high detail then go 131072 with 32 DM ratio and wait until the fourth of July.