I'm not sure myself about the "Heightfield (in RAM)" but I just made a 1024x1024 map and just checked a preview of my work with OFP. No errors or anything
I did note that when you click on "new map" use "Design/inflate (black)" and after clicking next, the map size is 1024x1024 (x=16 Y=16), to make a smaller 512x512 use X=8 Y=8
then click okay
Design map brush page, I'm winged my way thro it and can't really give any advice yet.
BTW, here is the 1024x1024 I made, just import it to WrpTool.
http://www.leondus.com/egroups/0a.rar 3.7mb