How can I make a heightmap with a sculpting program that's usable with Unity Terrains?

I want to sculpt on a plane in mudbox and extract a greyscale from it to use as a heightmap (mudbox has a displacement map that looks to me like it should work). After exporting the displacement and converting it to RAW format in another program (because apparently that's all that Unity's terrain will accept), I get results that look not$$anonymous$$ng like what I sculpted and have tons of artifacts. I've included an image.

When I do the same t$$anonymous$$ng by downloading a heightmap from the internet and convert it to .RAW in the same program I used to convert the mudbox image it works. The only difference I can see in the process between the two is that when I use the internet heightmap (w$$anonymous$$ch was strangely 257x257), the import heightmap dialog box in unity auto-populates the width and height fields to be 257 and 257, but when I use my mudbox-generated-heightmap, w$$anonymous$$ch is 1024x1024, it autopopulates the width and height field to be 1 and 1. I've of course tried changing these to 1024 and 1024, but to no avail. Please help!