As an update to the poster-size map: the image overall is coming along nicely. I've got the terrain sculpted, and although I don't feel great about my color gradients, they're drafted. Here are a few interesting limitations I've bumped into:
Stability hasn't been a problem yet for GIMP or WILBUR, though a few quirks have popped up. The file size for my land sculpt is a little over 2 GB, but when I have it open and am editing it, the bottom of my GIMP window reports a larger file size (the RAM it's using, perhaps?). When this size gets over 9GB, I can't save the .xcf file until I delete some layers and reduce it below 9GB (there's plenty of space on my hard drive). Routine operations in GIMP are a bit sluggish, and WILBUR takes a painful amount of time to do erosion effects, especially incise flow and erosion cycle.
The output jpg file is 45 MB, which Photobucket doesn't seem to accept, despite their claims to having no upper limit (The uploading bar fills up, but it claims the upload failed after 10 minutes). I have to shrink it down to about a quarter of its size to post it on Photobucket or email it on Hotmail. I haven't tried using my gmail account for it yet.
Gaussian Blur doesn't work so well at this size. I tried using it to soften some of the land textures in prairie areas. It did the job, sort of, but when I did my bump mapping, it gave the terrain a terraced look, which wasn't what I was really looking for. The smudge tool didn't work well either due to the large brush size I had to use; it also produced some small linear jumps that show up in the bump map. Running the blurred image through WILBUR's precipitation-based erosion about 10 times made these problems better, but didn't entirely eliminate the issue.
All in all, however, the process seems to be working. Here's a 1/4 size of the image; please feel free to point out any issues, especially with the colors (I have a color vision deficiency which sometimes makes it hard for me to apply and balance color gradients the right way). There are two areas in the map that are desaturated of color, one in the water and one on land. That's deliberate.
3rdLandSculpt_zpsmkrdox5i.jpg