Hi Andrei, you can use the Transform > Deform > Flow command to warp from an unwrapped UV plane onto a 3D surface already, that's pretty much what it does.

To make the unwrapped version, there is no one direct simple width and height that really maps to a warped surface, but often times the length of a median line can be a good one to use to minimize warping. If you extract out curves near the middle of the surface you can use the UnwrapCurve plug-in to make line segments of their equivalent length: http://moi3d.com/forum/index.php?webtag=MOI&msg=5136.1

- Michael