Historical Note (Sept 6, 2009): Shortly after Warhammer Online went live, it became apparent that one side had, in general, a much higher population than the other. Real numbers were impossible to come by, as the game masters (EA/Mythic) were tight-lipped in regard to balance. As I found playing the game more and more disheartening, I instead shifted my energy to writing a few quick Python scripts to parse the guild population pages on the Warhammer Herald in order to produce accurate relative population figures. I kept this list updated multiple times per day for less than a week, during which time it received tens of thousands of hits. Then abruptly, as noted below, Mythic changed the guild page formats to no longer show exact guild populations. They awkwardly reduced the value of their product in order to prevent collection of further data.
These numbers are guilded populations only, as per the official guild search function on Mythic's site. Please note that these numbers do not indicate the numbers online - they indicate the total number of characters that are in a guild (it's a strange metric, but it's all I could get without loading a million pages from Mythic's site, which they wouldn't like).
UPDATE: As of late Wednesday, September 25th, EA/Mythic has changed the display format of their guild search page. It now shows only ranges (1-25, 25-100, 100+) instead of precise guild populations.
Last Update: Wednesday, September 25, 2008, 10 AM
Overall - Order: 69811, Destruction: 102576, Ratio = 1:1.5
RvR - Order: 10646, Destruction: 22717, Ratio = 1:2.1
RvR-RP - Order: 1498 , Destruction: 2251 , Ratio = 1:1.5
RP - Order: 9829 , Destruction: 10446, Ratio = 1:1.1
Core - Order: 47838, Destruction: 67162, Ratio = 1:1.4
Anlec - Order: 1402, Destruction: 2275, Ratio = 1:1.6
Avelorn - Order: 1153, Destruction: 775 , Ratio = 1:0.7
Averheim - Order: 2075, Destruction: 2411, Ratio = 1:1.2
Azazel - Order: 2074, Destruction: 2334, Ratio = 1:1.1
Badlands - Order: 2459, Destruction: 2772, Ratio = 1:1.1
Be'lakor - Order: 514 , Destruction: 518 , Ratio = 1:1.0
Bechafen - Order: 1103, Destruction: 3386, Ratio = 1:3.1
Bretonnia - Order: 1736, Destruction: 3018, Ratio = 1:1.7
Chaos Wastes - Order: 1498, Destruction: 2251, Ratio = 1:1.5
Dark Crag - Order: 1919, Destruction: 3210, Ratio = 1:1.7
Darklands - Order: 2655, Destruction: 3773, Ratio = 1:1.4
Drakwald - Order: 669 , Destruction: 1207, Ratio = 1:1.8
Drifting Castle - Order: 1551, Destruction: 2077, Ratio = 1:1.3
Gorfang - Order: 805 , Destruction: 1427, Ratio = 1:1.8
Grimnir - Order: 260 , Destruction: 703 , Ratio = 1:2.7
Heldenhammer - Order: 860 , Destruction: 1402, Ratio = 1:1.6
Hochland - Order: 1230, Destruction: 3539, Ratio = 1:2.9
Iron Rock - Order: 2146, Destruction: 3238, Ratio = 1:1.5
Ironclaw - Order: 1772, Destruction: 2891, Ratio = 1:1.6
Ironfist - Order: 2262, Destruction: 2681, Ratio = 1:1.2
Kislev - Order: 417 , Destruction: 751 , Ratio = 1:1.8
Kragg - Order: 612 , Destruction: 726 , Ratio = 1:1.2
Lucan - Order: 241 , Destruction: 402 , Ratio = 1:1.7
Lustria - Order: 194 , Destruction: 276 , Ratio = 1:1.4
Marienburg - Order: 85 , Destruction: 352 , Ratio = 1:4.1
Marius - Order: 163 , Destruction: 426 , Ratio = 1:2.6
Middenheim - Order: 268 , Destruction: 678 , Ratio = 1:2.5
Monolith - Order: 1963, Destruction: 2865, Ratio = 1:1.5
Ostermark - Order: 2280, Destruction: 2731, Ratio = 1:1.2
Phoenix Throne - Order: 3345, Destruction: 3480, Ratio = 1:1.0
Praag - Order: 1896, Destruction: 3617, Ratio = 1:1.9
Rakarth - Order: 262 , Destruction: 365 , Ratio = 1:1.4
Red Eye Mountain - Order: 2642, Destruction: 3657, Ratio = 1:1.4
Reikland - Order: 266 , Destruction: 231 , Ratio = 1:0.9
Saphery - Order: 517 , Destruction: 304 , Ratio = 1:0.6
Sea of Claws - Order: 512 , Destruction: 993 , Ratio = 1:1.9
Sea of Malice - Order: 986 , Destruction: 1567, Ratio = 1:1.6
Skavenblight - Order: 419 , Destruction: 1002, Ratio = 1:2.4
Skull Throne - Order: 2923, Destruction: 3353, Ratio = 1:1.1
Sylvania - Order: 2471, Destruction: 3501, Ratio = 1:1.4
Thorgrim - Order: 852 , Destruction: 1116, Ratio = 1:1.3
Tor Achere - Order: 182 , Destruction: 365 , Ratio = 1:2.0
Tor Elyr - Order: 616 , Destruction: 653 , Ratio = 1:1.1
Ulthuan - Order: 2538, Destruction: 2872, Ratio = 1:1.1
Ungrim - Order: 1959, Destruction: 2967, Ratio = 1:1.5
Volkmar - Order: 3003, Destruction: 3512, Ratio = 1:1.2
Vortex - Order: 1960, Destruction: 3459, Ratio = 1:1.8
Wasteland - Order: 1176, Destruction: 2248, Ratio = 1:1.9
White Tower - Order: 1822, Destruction: 1529, Ratio = 1:0.8
Wolfenburg - Order: 1955, Destruction: 3795, Ratio = 1:1.9
Wurtbad - Order: 1141, Destruction: 2895, Ratio = 1:2.5