Generated heightfields do have a base finite pixel resolution although like imported heightfields they are enhanced with procedural detail by default. The error message seems to indicate that the heightfield may be generated at 32bit precision, requiring twice the memory of a 16bit and thus 2x 860MB or about 1.7GB (1,764,000,000 bytes). Even if you had enough physical memory (RAM) for this operation it is almost certain that Windows could not allocate a large enough contiguous block to allow it.
- Oshyan