You can work out something rough by knowing:
- Toggling the grid lines in the Heightmap editor enables you to see specific cells (Each box is a cell)
- There are 64 grids (box bits), therefore 64 cells, in each area of the 'Overview Window'
- As this show, the Overview Window is an 8x8 matrix
- That means there must be 512 (64 x 8) cells along each side of your 1024x1024 square
If that's true, then every 2 pixels are a cell; which seems stupid. If you take a screenshot of the heightmap editor with it's gridlines on, each grid/box takes up 10 pixels by 10 pixels.
So the conclusive answer is - I don't know exactly; but the Wiki tells us:
Each exterior cell is 4096 units by 4096 units or 192 feet by 192 feet or 58.5 meters by 58.5 meters. Each vertex (in the landscape terrain) in an exterior cell is 128 units apart - the same height roughly, as a human bi-ped, or about 6 feet.
The CS says there are 11,926 cells (in a 134 by 89 grid) in Tamriel (although a lot of them won't be accessible, due to the Region boundaries).
I'm not sure if that helps at all or not, but a 1024x1024 heightmap will mean you need a 1024x1024 map to go ingame; and Tamriel (all of the map) uses a 2048x2048 map (although some of this isn't viewable, and it shows bits of the border regions anyhow).
Good luck. I'm not sure if I've actually proven anything at all; just quoted numbers to you. Hope it helped, somehow!