If you're on a ladder, the best you can hope for is a three-by-three
foot square assuming a normal adult armspan. This gives you about five
feet on diagonal (well, 4.25'), and is probably as far as you'd want to
stretch your arm overhead from your center of gravity.
Three-by-three squares also give you the opportunity to skimcoat at
least one taped joint in each square.
If you're on a scaffold, I suspect the areas might be larger, but I've
never tried that method so wouldn't hazard a guess.
Here\'s some of my work:
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