I have a feeling that they nerfed Sargeras' true size because of both the in-game engine, as well as his imprisonment slowly shrinking his ass as he's getting pushed back to the Seat.
I mean, as he's getting pushed back, you can see that his index finger-tip's relative in size with The Vindicaar (Which shouldn't be the case at ALL, unless the Vindicaar's just that big).
At the very least, Sargeras SHOULD be relative in size with Azeroth, and that's sword wound SHOULD span far beyond just a simple zone. Half of Kalimdor + Part of the sea below should have been fucked up by Sargeras' blade. But, it wasn't.
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So, unless he was shrinking, or got nerfed, or he was still manifesting into physical being, Sargeras' size is VERY inaccurate.