These two maps should be in scale to each other. The Wilderlands of High Fantasy is pretty dang tiny compared to the World of Greyhawk. Imagine how awesome it would be to have Greyhawk fleshed out with maps at 1 hex = 5 miles, full of Wilderlands style encounters. That'd be a hell of a lot of work.
This comparison turned out pretty much like I expected. The Flanaess is mostly a mirror image of Europe with Lake Superior dropped in the center.
Here are the figures I was working with, if you want to check my results and/or expand upon them:
Flanaess 4800 miles E-W , 3600 N-S
Wilderlands 780 miles E-W, 1020 N-S