How big is Bahamut?

[Nutz]

The thing I don't really understand about primals is, they were something before, weren't they?

I'm not sure that's always the case. Certainly for Saint Shiva and Bahamut it is, but it almost certainly isn't the case for King Moggle Mog XII. For the others I'm not sure. Perhaps there's lore supporting their corporeal origins, but I've always taken them to be essentially demigods made real through aether and devotion, regardless of having ever existed before.