ahh, I thought they were basically the same phone except for cell tower chipsets.
The Droid for Verizon does use wallpaper dimensions of 960x854 as I've made a few. You also can visit the gallery here and right click any of the wallpapers in full view mode and check the properties to see the size too.
The same wallpaper works fine whether I have 3,4,5,6 or 7 desktops setup. The wallpaper just doesn't scroll as much when I move from screen to screen.
On the Milestone if you install home++ do you get more than 2 desktops?