If you watch the end of an era video, you can see Bahamut briefly against the Limsa Lominsa backdrop. He is roughly right above the city, so using this as a scale it definitely supports the claims (and from what we saw in the Coils) that he is as big or slightly bigger than most Eorzean cities head-to-tail. Wingspan is much larger. Overall, he's monstrous in size.

Bahamut in life may have been much smaller. Remember that the primal-version is represented as how the Mercydian dragons imagined him (as is with all primals). It's true that Midgardsormr's children are definitely larger than most dragons we will encounter, but Bahamut's sheer size may only be due to the fact that he was reincarnated as a primal. Likewise, he may have always been unnaturally big. 3.0 should give us more story into the origins of dragon-kind and how they all relate to each other.