I experimeted with higher resolutions, when I did my ANW re-paint. Originally they were made in 2048x2048 which would give just slightly less than 1m per pixel which should be VERY good. However, what I didn't take into account is the quality of the texture itself. In the end it turned out that sizing it down to 1024 did not change the looks of it at all, only sizing it down to 512 showed some extremely mild quality reduction to the point of only a sand grain counter would notice, a worthy notice to mention is that tile size didn't affect my FPS AT ALL even though I had only 1GB system ram/128MB gfx ram at the time. At that point I realized that with current satelite imagery available to "mortals" such things are rarely worth the effort, or in other words, areas of such quality are very rare on GoogleEarth.
Now as for the mega tiles, to use 64x64Km tile and to get 1mpp you would need 64000x64000 sized tile, that's 411MB, I can't even imagine how long would the terrain load with several of those. However I must admit that terrain variety could dramatically improve with that. The second problem is the terrain mesh, or in our case HFD, which is not very detailed, how would a tile, which would basically be a 2D image of the area, fit onto the HFD...
However don't take my skepticzm as a downer, I'm really interested in your ideas and would like to see how and if you can pull it out