How large is Ghidorah based on this Cave Painting

He would probably be close to his size in the Heisei saga. 

Ghidorah was 492ft in the heisei series. Godzilla was 328ft. So if G14 is 350ft in the monsterverse and if we are going with the accuracy of ancient cave paintings, my guess would be around 550ft. That includes wing height. That's my guess based on the painting. King Ghidorah has always towered over other Kaiju's. 

^He's hovering in the air, which could lead to overestimates.

Probably little bit taller than Heisei Ghidorah.

To me, Ghidorah doesn't look like he's hovering. I think there are in the foreground and there are mountains in front of them, to show size and mass. Godzilla doesn't look like he's in full figure so I assume its the same with Ghidorah. You don't see each others feet. but we do know Godzilla is 350ft. so I only assume based on my observation that Ghidorah is around 500-550ft.

Yeah, it doesn't look like he's hovering. There's just mountains and images of people fleeing/dying in the foreground of their bodies.

I doubt the cave painting image is anything to take too literally, but with Godzilla standing 108 meters, the Ghidorah in the painting seems to indicate something larger than 150 meters--But I'm eyeballing something potentially not worth putting much stock in.

150 meters or so sounds like a good baseline, taller than the tallest film Ghidorahs to date whilst not being too ridiculously tall, like Legendary Godzilla himself.

^

Maybe slightly taller than 150 like 155, 158, or 160.

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I just know they are going to make him ridiculously big. 150 meters isn't anything new for King Ghidorah. He has been that size before. Legendary likes to give us depth of how big a monster is. Pacific Rim, G14, K:SI, they loved bragging how gigantic these monsters are. Im guessing they are going to try to feature Ghidorah as the biggest, baddest thing to ever show on screen. He is going to tower over Godzilla and other structures. I just see a pattern in their films.

Actually guys, the All Toho Kaiju encyclopedia book from Toho actually confirms heisei ghidorah was 140 meters. This book is the most up to date book. So, I look for Legendary ghidorah to be 148+ meters. 

^Oh, damn. I need to step up my game. Thank you for clearing that up. Ya think you know your stuff and then this gets dropped on ya. I still stand by my statement. An extra 8 meters we probably wont notice on ghidorah, so they must go higher than 150 meters. If I were a gambling man I would put money on it.