So, I just did T5 to help a friend out and used the 100 x 100 yalm grid square to measure Bahamut's palm (from the 
end of the wrist to the 
start of the middle finger), which ends up at roughly 130 yalms. Using concept art to measure how many palms he is high, I get a 
very rough ballpark of 9,360 feet (1.7 miles) as his height.
Now, at 9,000 feet, we're talking 3.3 
Burj Khalifas. We're talking almost the size of 
Mount Olympus.
I can't help but wonder how much wiggle room there is between the various sizes of his various appearances, lol.