In the original Greyhawk Folio, Gygax listed Ull's size as such:
"The territory comprises over 800,000 square miles, including the hills that separate the Ulsprues from the Crystalmists."
Then in the Greyhawk Boxed Set, Gary repeats the text on Ull, but changes the units of area from miles to leagues, a measurement that he pushed heavily in the boxed set tables, 1 hex is 10 leagues wide which is 30 miles:
"The territory comprises over 90,000 square leagues, including the hills..."
Not knowing how to do square distance of anything hex shaped, I deferred to Gygax's judgment here, but checked the math on an online calculator and got these results:
800,000 sq mi = 67,121 sq leagues
90,000 sq leagues = 1,072,677 sq mi
Okay so both are off, but its in the same balk park. Now what's screwy is there is very few other nations in these early Greyhawk books that give an amount of area. Ull is special here because it's remote? So how big are these numbers for context? Well on Earth, 800,000 sq miles is between the area of Mexico or Saudi Arabia. On the other end of this Gygaxian measurement, 1 million square miles is like the size of India or Argentina. For comparison the USA is 3.5 million sq mi. So seeing these examples, it's obvious Ull is not that big. Check out this map overlay I found online. (sorry I can't find the author to give credit, feel free to shout out in the comment section) Needless to say if Ull were the size Saudi Arabia, it would presently be half the area of the old Baklunish Empire.
Post-Gygax sources, From the Ashes and the Living Greyhawk Gazetteer, both drop the area of Ull from its text, I presume because the writers finally recognized that Ull cannot possibly be that large and Gygax goofed. So again, for Ull to be that big it would have to claim/conquer the Plains of the Paynims, the Dry Steppes and more west possibly. The Living Greyhawk Gazetteer mentions Ull's northern border is in flux with the Paynims, but is usually within 20 leagues of Ulakand, the capital. This is less than a hex so that checks out. Ull does not claim much of the Plains of the Paynims, nor does it encroach on the Dry Steppes because the text says Ull ends at the hills separating the mountains.
So how big is Ull really? My non-scientific eyeballing and using a calculator method has Ull (which is roundish) at roughly 300 miles by 360 miles or 108,000 sq mi. This puts in the category of New Zealand or Burkina Faso. Much more modest. I guess it'll have to do (for now). All of this doesn't matter of course, but explaining the minutiae of the setting is part of Greyhawk's charm. Until next time, see you online!