Here's Darlene's original map for the World of Greyhawk. These hexes are 30 miles across. Note that Gygax is said to have invented this version of Oerth and the Flaeness for publication - his original campaign grew from the environs of Greyhawk Castle outward rather than being a top-down vision from the start.
Here's a later TSR map detailing a smaller area of that map (Admundfort is visible in each) using 20 mile hexes, which doesn't fit any kind of system (perhaps because hex-crawl campaigns were already being forgotten?). Nevertheless, this is useful for showing the kinds of details that are hidden in 30 mile hexes but revealed by just a little zooming in.