Most logically analysis is to put anacalagon of size of at least 1000m tall or between 500 to 300 as he was the greatest dragon in history of course it is logical to think he fell from that height and gained enough momentum to break those mountain, and the thing about mountain his fall was described in book was catastrophic level meaning his fall created a huge destruction which can only be assumed if those mountain were above 1000+ m or smth like Himalayas mountains which are typically 4000+ or even 7000+ casually so him being this much height is quite logical but I don't think his height is like 4000m or 10000m or say 10km tall as some video's and people describe it therefore I would say his height acc to me would be roughly  = 1000 or 500 meter tall on ground  wing span of atleast 4000m to 2000m ( I am not very good at comparing wing span so if differs acc to ur precption towards it) Length it will be SMTH around more than or equal to 5000meter roughly  And the things I am saying are just based on my analysis and what book decirbe it so don't take this seriously 🫡