From:  Michael Gibson
1104.16 In reply to 1104.15 
The map will be at a fixed resolution for each surface, like a grid of 64x64 sampled points.

So the broader your surface, the less individual detail it will have since those points will be spread out over a larger area.

But your object doesn't have to be a solid, it can be an open surface or just one sheet - but any trimming information on the surface (which is typically added during boolean operations for instance) will get ignored and just the underlying surface will get converted to the sculpt map.

- Michael