Gosh http://www.gamesas.com/index.php?/topic/1264541-skyrimoblivionmorrowind-heightmap-comparison/page__view__findpost__p__19243939 on these forums and I don't get called a genius (though maybe the italics mean something else).

And apparently you can play the rest of Tamriel if you find a secret pass through the mountains.

Btw I wonder if the Cyrodiil LOD was included as it can be seen as part of the main quest - when you're up at the Throat of the World you can scramble to the very peak and have a clear view south. The weather was such that I couldn't see the white tower as it was obscured by hovering clouds, but I could see further over the top of it - as far as the southern sea. There are other quests too (like a statue near Solitude) where you will float a mile or so above the worldspace, able to survey everything, and it would look daft if the LOD landscape suddenly disappeared at the edges of the actual heightmap.
Ok back to my point ... Is Skyrim around 6 kms ? and not 3.8 ? so more or less like 1.8 m x 3800 ?
Yes. 3800 pixels corresponds to about 6.8km distance.
how large is the brush for painting terrain ? I mean how large is a square of texture minimum ? covers one whole pixel from the heightamp or more? or Less?
In the game and CS there's one texture point placed for every heightmap point.
But TESAnnwyn exports every texture point from the ESM/ESP - and there is extra detail in there:
There are 32x32 heightmap points per cell.
There are 34x34 texture points per cell (and up to 9 layers).
So the texture files exported by TESAnnwyn will be slightly larger dimensions than the exported heightmap.
e.g.
Oblivion's Heightmap: 4288 x 4128
Oblivion's Texture Map: 4556 x 4386
If you have a texture placement image from another heightmap and decide which colour will refer to what texture, you'll need to enlarge the image by a fraction of 17/16 (1.0625) - and make sure when you enlarge it that it is not interpolated by any algorithm - so the colours remain exact; because each colour refers to a texture FormID. I'm pretty sure you've done all this in the past with projects like Dune?