Now this is exactly the sort of thing I was looking for!
A single identifiable letter, big enough to be shown without being all blocky. My computer is playing up a bit and photoshop wont save for web, so I've turned the quality down a bit on the image, but it's still as good as I wanted it to be. It's an upper case X because... well because that's what it looks like to me. If I find a better X it might be used as a x instead.
This letter is on a patch of tarmac at Vandenberg air force base. It says, "Don't land here!"
Here it is.