Here's a screengrab of Bahamut flying past Limsa.



It's hard to tell exactly how big he is because he flies behind Limsa and there's no indication how far behind he flew (could be 300 fulms, could be 30,000 fulms, would contribute a big difference in estimated size). However, since he's farther behind Limsa, that puts a lower bound on his size as at least as big as Limsa castle and possibly much bigger. Multiple hundreds of meters tall at least.

An additional size data point could be the recreated Dalamud inside T11. Not sure if it's a full size replica (which would imply the the unfurled underground Binding Coils expanded larger than Dalamud itself) or a scale model. The platform Kaliya appears on is the "inner ball" that a curled up Bahamut with wings folded fit inside.

Another thing to note, Bahamut also flies from Carteneau to Limsa and back in a matter of seconds. If we maintain that Limsa > Gridania is 1600 km and Limsa > Carteneau is half that length, then a round trip would also be 1600 km. I didn't put a stopwatch to it, but he must have been flying at multiple hundreds of km/sec, which far outstrips the escape velocity of Earth (11.2 km/sec). Food for thought.