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-New Information:

With help from /u/bakb0ne it was brought to my attention that on page 328 it states that, "the once-tiny robot [Leopardon] had grown to a height of almost a hundred meters." So we finally got confirmation that Leopardon is as tall as I thought he was. Well, I guess we always had it. I just forgot. Haha

/u/FrateTrain94 also told me that on page 266, in reference to the Orb of Osuvox, it states, "According to the artifact's item description, it could create a spherical shield around itself, with a circumference of up to a half a kilometer." Now if one does the math that comes out to a radius of 80m. Now, that wouldn't make much sense considering the fact that Leopardon is supposed to be near 100m tall and Kiryu is supposed to be twice his height. So /u/FrateTrain94 and I guessed that Ernest Cline must have meant that the Orb had a diameter of half a kilometer, which would come to a radius of 250 meters. Perfect. Thanks, /u/FrateTrain94, for making me do my first math problem in years (cough jerk cough).

Seriously, though, thanks /u/bakb0ne and /u/FrateTrain94. Couldn't have done this without you.

-Old information:

In the book it states Sorrento's "tiny robot instantly grew in size until it stood almost as tall as Castle Anorak itself, twice the height of the “giant” robots that Aech, Shoto, Art3mis, and I piloted." Also in RPO it says that Shoto's mech, Raideen, "stood several heads taller than both of them." Indicating Aech's RX-78 Gundam and Art3mis's Minerva X. With these lines I was able to estimate that Raideen stood at about 100m while I bumped RX-78 and Minerva X up to 75m.

Last comes Ultraman. Parzival described his interaction like this: "I now stood face-to-face with Sorrento’s mech, matching its height and size almost exactly." So I made Ultraman 199m tall.