I think the sizes in their respective pages are wrong. Here's why:
According to the official guide TTGL is 10^25 times bigger than Gurren Lagann. It seems like some people think GL is 10 mts (I've even hear people saying it is 5 mts) however Lagann is at least 1,50mts and GL is 10 heads tall so 15 mts should be its height.
If we multiply 15 x 10^25 we get 150,000,000,000,000,000,000,000,000 mts which is equal to 15,855,012,510 lightyears. Or lets round it to about 16 billion ly.
Even if we suppose that GL is 10 mts tall we still get: 10 billion ly not 10 million.
About STTGL, to my knowledge there's no "official size" in the guide. The guide only covers up to TTGL. I don't know if a new guide was made after the production of the 2 movies but I've never seen it.
So I think the 52.8 billion ly size came from a (faulty) calculation made using 5 mts as GL base size.
If you use 5 mts to calculate the size of TTGL you get this:
5 x 10^25 = 50,000,000,000,000,000,000,000,000 mts which is equal to 5,285,004,170 ly
Then I guess they multiplied 5,285,004,170 by 10 and boom! You get 52,850,041,700 or 52.8 billion ly.
According to the previous calculation STTGL is 10 times bigger than TTGL. But again, I don't think this was stated in any official info released. I think the "official statement" was that its size is immeasurable.
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But we can at least get an idea of its size:
For what we can see in the anime, Granzeboma compared to STTGL is about the size of its "big toe".
So just the sole of its foot is about 5 times the size of Granzeboma. If we estimate STTGL size using its foot size as reference we get:
Granzeboma size = 15,855,012,510 ly (same as TTGL)
STTGL foot is 5x Granzeboma = 79,275,062,550 ly
A person's height is supposed to be roughly 7 times the size of its foot so 7 x 79,275,062,550=
554,925,437,850 ly! Around 555 billion ly.
This method of multiplying the foot size by 7 is not perfect so give or take some million ly.
We can also try to use the screen capture from the anime to calculate:
If you draw a line of the same height of Granzeboma in this pic and keep copying it until you reach the top of STTGL you get about 20. However, in this picture STTGL has its legs spread which affects its size making it look a bit smaller (try spreading your legs in front of a mirror and check how your height is decreased compared to when you stand straight).
But lets ignore that for now and say that STTGL is 20 times bigger than Granzeboma.
15,855,012,510 x 20 = 317,100,250,200 (about 300 billion ly)
So the best approximation is that STTGL is between 300-550 billion ly tall.
Unless I'm wrong with this I'd to change the sizes in the TTGL and STTGL pages.