NPCs specifically referring to Ansur as Wyrm. And in fact several times even called him Great Wyrm.
Meaning Ansur is very old dragon (at very least 1000 years old, over 1200 years as a Great Wyrm).
The size of him however suggest he is at best just an adult dragon.
Really breaks the immersion. You descend there, expecting to meet truly massive ancient dragon. And game just throw at you adult-size dragon, which is hardly that impressive, as there thousands of adult dragons across Toril.
